I would be shocked if the problem is anything other than a worn out cassette. Is the chain actually slipping when you pedal or is it just rubbing?
Damn, you're going to cling to that one to the bitter end, aren't you? It was a reasonable first guess, but since then you've learned:
- It's a quality bike with only 1k miles on it.
- The cassette is not visibly worn.
- Making cable adjustments helps the situation.
And you still think it's the cassette?! Do you work for a cassette manufacturer or something?
Even *if* the cassette is worn (it's not), it wouldn't be exhibiting any problems, because the chain has not been replaced. Cassettes get worn down by a "stretched" chain. As long as the stretched chain and the worn cassette stay installed with each other, they'll work fine because they match each other (at least until you get to an extremely
worn cassette). Only when you put a new chain on an worn cassette will you really start noticing problems. This leads to yet another argument against the worn cassette/chain theory: if the chain was stretched and wearing down often-used gears on the cassette, those gears
would still probably work fine; it's the little-used gears that might show problems until the stretched chain gets a chance to wear them down too.
At the moment, since operating the barrel adjuster changed something, derailer adjustment sounds like the most-likely problem. Occam's Razor here, people.http://parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur
is a complete, detailed, and easy-to-follow guide to do a full derailer adjustment. The OP should at least do that before dropping $50 on a new cassette+chain. If that doesn't resolve the problem, it could still be an adjustment issue, just one caused by a cable problem, or a bent derailer hanger, or something like that.
Also, suggesting 1,000-2,000 miles for chain replacement seems pretty extreme and wasteful, like changing your oil every 3000 miles. Sure, maybe check it ("Measuring Chain Wear")
every 1,000 miles, but only replace it if it's stretched over 1/16" per foot. And maybe not even then. My cassette is 10 years old and has probably 15,000 miles on it, and it's had the same chain on it for at least the last half of its life/miles. Yeah, both chain and cassette are both worn to hell (the chain is stretched 1/4" per foot), but they both still work fine together.