ok, cog is good. i pedaled up the biggest hill and walked back down, twice. then tightened the lock ring REALLY tight. turns out the lockring was never being tightened down enough by the guy at the bike shop. he was tightening it without removing the chain, and therefore could not put a lot of pressure on the spanner since it wasn't on straight (chain in the way). i borrowed his spanner and did it myself, with the chain off, and was able to get it like a zillion times tighter. thanks everybody.