Really? When I got my first classical guitar, a requinto (pint-sized model) in Mexico at age 11, it came with steel strings on it, and I changed them to nylon to be easier on my tender young fingers. From that time on, I was under the impression classical guitars could use both. 
But I’m extremely ignorant about guitars, really.
No way… The majority of classical guitars don’t have a truss rod in the neck or the robust internal bracing under the sound board to compensate for the much higher tension of steel strings… Steel strings will warp the neck and/or the sound board sooner rather than later if installed on a classical guitar…
+1 on the above suggestions to avoid Martin strings in the future, it’s not that they’re total rubbish it’s just that there are better options available… As for your tuning woes often a string or a chord sounding out in certain voicings and positions is due to an intonation problem, but since the intonation on classical guitar isn’t really a user adjustable thing I’d disregard that for now and double check the installation of the G string… Often times if you’ve not tied the knots at the bridge and machine head just so, they can be very slowly undoing themselves under tension and slowly dissipating the decreased tension through the wound portion of the string leading to a frustrating refusal to settle and hold pitch… An unwound nylon G is prone to this as it’s the largest diameter unwound string and the hardest to get a good knot to hold on… If it just won’t cooperate you might try very lightly roughing up surface of the end parts of the string where you will tie that knots with some fine grade sandpaper to allow for extra friction in the knots and a more stable install…
Intonation going out can be indicative of some nasty things like neck and/or soundboard warping so if the problem persists through several sets of strings you may want to have the guitar looked at… Taiwan’s relentlessly high and fluctuating humidity makes it a hard life for most stringed instruments, but I think you’ve most likely got a string problem more than anything…
EDIT: If the most recent set of strings is of different tension, gauge or perhaps even brand to previous sets, it’s conceivable that the string is no longer seated correctly in the nut groove which can dampen vibration leading to dull tone and throw off intonation…
big
specialized
A footnote to 'tron’s point… well built classical guitars can often handle light steel strings, e.g., tens. Most luthiers build for a given weight of string, esp. classical guitars. Production line guitars aren’t nearly as sensitive.
The most remarkable “cheap” acoustic guitars I’ve come across recently have been the Epiphone Masterbilt (no “u”) series. You can pick them up for USD300 on Craigslist, and many I’ve played have been better than the current Martin D-28.
Thanks for the links, maunaloa!
That D’Addario name looks extremely familiar. I’m sure I must have replaced a couple sets with those in Mexico or something.
EDIT: That ‘big’ link says they don’t ship internationally via their website; the other has a minumum of $40 shipping. 
D’Addario is all I play. I buy them by the 10 pack.
The D’Addario coated steel acoustic series, e.g., the EXP series, is worth the extra money. Very slightly less bright when first strung, but they stay that way … for a year.
I assume a 10-pack is ten sets of six. Si? Where do you get yours? Online via those links? Is it cheaper that way, or do they just not have the ones you want in Taiwan?
Since I’m just learning about this, I might as well fire away with more stupid newbee questions if y’all don’t mind. I looked at the Hannabachs on one of those links, and they had low, medium, high, and super-high tension sets, within the silver special edition grouping. What would I want for beginner’s classical? I’m not overly worried about the toll on my fingertips, btw. What are the pro’s and con’s of these differences in tension?
EDIT – I called Tony’s. They carry only one tension, Medium-High in the Hannebach 815 special edition silver, NT$450 after discount for a set of six. That’s the same price as at juststrings.com/hannabachcla … ar815.html, before adding US$40 
shipping to the latter. Tony’s also has a cheaper Hannebach, the 8001’s in MT or LT, NT$350. It’s all Greek to me. Oh, and they sell single strings, subject to availability, but only GHS and Thunder brand, NT$30. Are those locally produced crap, or what?
Order from these guys.
http://www.guadalupecustomstrings.com/aboutOurStrings.htm
They’re all I’ll use these days.
[quote=“sandman”]Order from these guys.
http://www.guadalupecustomstrings.com/aboutOurStrings.htm
They’re all I’ll use these days.[/quote]
That’s a very interesting site, Sandman! When I get a little better I do think I’ll check them out, thanks! Right now it would be pearls before swine, though. 
Your guitar isn’t bad. Its worth it. Treat yourself. I can’t recommend the wound nylon strings highly enough. Like nothing you’ve ever used before.
The guitar isn’t bad. The Martin strings aren’t bad. I think the problem lies elsewhere. Get someone to look at it.
Thanks, Doc (and everyone)! I checked with an online tuner, and found that my guitar is tuned a full step down. I brought the whole thing up a step for a couple hours, played it a lot, brought it back down a step (I like the richer deep bass sounds), played it for an hour, and now the problem is drastically reduced. 
I think jimi and buttercup were right about it needing to settle – I just didn’t realize it would take so long. I had already stretched each string about three times when installing, but that wasn’t enough. Bringing the tension up and then back down appears to have helped the strings settle much more quickly. I’m still not in love with the strings, especially the G, but I’ll go ahead and play these strings for a month or two before changing them. I may reinstall it like plasmatron suggested, just to see what happens.
It’s nice to have recommendations of what strings to try next, thanks!
I buy D’Addario sets from whomever has the cheapest price, including shipping. I have a collection of guitars, electric, acoustic, and semi-acoustic (aka electric acoustic, or semi-hollow body electric). My playing style involves alot of overtones (aka pinched harmonics, think ZZ Top, “LaGrange”), and steel strings lose the top end first, and suddenly. Thus, 10-packs of tens (refers to .010" high E string) and elevens (i.e., .011" E string). I break nines and smaller like biting a cracker.
I told you Noel, we ought to bring our guitars to one of Maoman’s chillaxin’ dealies up there in Lotus Hill.
You’re trying to play new nylon strings tuned a whole step lower? That’s the problem right there! Even heavy steel strings take ages to settle like that. I play fairly heavy steel strings on my semi-acoustic – 0.17 for the top e usually – and often tuned a whole step down, and they take a while to settle if I don’t tune them to concert pitch for a while first.
Still, though, you should seriously consider some of those Guadeloupe fellows, especially since you’d be supporting “the little guy,” of whom there aren’t nearly enough, IMO.
And a decent tuner, for god’s sake! It’ll be the best NT$1,500 you’ve ever spent in guitar terms.
Good to know, thanks!
I might, if and when I get smart enough to know what I would want to order from them. Right now, I’m like uh, uh, do you have strings?
Yeah? Is there a particular brand or model people here like?
I’ll tell you tomorrow once I’ve had a look at the brand. As guidelines, though, you want a backlit (very important!) quartz display (cheaper) that shows up as a needle, like an old-fashioned analogue device that you might have seen as a VU meter on your old dad’s tape recorder or something. Avoid the kind that look like credit cards with a row of little lights on them – they’re crap.
Its also great if it can clip on to the headstock of the instrument – saves fiddling about trying to balance the thing on your knee and get a reading while simultaneously twiddling the tuners.
I got mine from a friend who bought it in the states, but they’re available online and I’ve seen other people here using them, so they could be available here also. Inexpensive.
Mine’s is something like this. They clip on and sense the frequency via vibration, so it doesn’t matter if the TV or record player is on loud, it won’t affect the tuning ability.
And that one, at 11 bucks, is not exactly mind-bendingly expensive.
http://www.guitarbitz.com/1183.htm
I’ve got a NT$5,000 tuner at home that’s gathering dust (no, you can’t have it!) because the little cheapy one is just fine and dandy, weighs nothing, and slips into your pocket.
Cool, thanks. I know Tony’s has some in the $500-700 range, although I haven’t actually looked at them. I’m not yet convinced that someone with a keen ear for intonation needs one (especially if they only play solo at home), but it might still be convenient for that rare occasion when the tuning is elusive, or as I grow older and deafer. 
I’ll bet they’re really good for people who need to show up at a gig with everything tuned and ready to play with others, though. I mean, besides the strings being in tune with each other, you need to be in tune with other players. At the moment, I don’t.
I have a tuning fork. But I don’t use it much. I use a Boss tuner.
Yeah, I used to think that, too. Prepare to be surprised at just how keen your ear is – or isn’t. 
Seriously, the difference to your overall sound with the micro-tuning possible with these things is VERY noticeable, whether you’re playing along with a record or other people, or just playing with yourself.
I bet there aren’t many guitar teachers in ANY style who don’t recommend tuners these days.
I’m certainly keen to try it, thanks!