+1 on chain stretch. shine a light behind your chainring and look at how the chain sits in all of the teeth.
also, take the chain off, spin the crank and look at the space between the chainring and chainstay and make sure that it's constant thru the rotations while you're checking things out. might be a good time to adjust the chainring if it needs it.
is your entire drivetrain 1/8"?
first time i threw my chain, it was on a mixed drivetrain. 2nd time i threw it, the cheap chain was majorly stretched. i bought a chainwear gauge for a couple of bucks and check my chains every few weeks now. most likely overkill, paranoia and OCD on my part, but i don't mind.