Great State of Varmint
Join Date: Sep 2008
Location: Dante's Third Ring
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Check the angle of the derailleur's two pulleys (wheels, tension-roller and jockey). Are they in the same plane as the rest of the bike's centerline? Or are they angled by comparison? If they are angled (probably inwards), you have a bent hanger. Depending on your frame, this may be fixed by bending it back to where it belongs. Or by bending it's derailleur-hanger. Or even replacing the hanger, depending on how off it is.
Rather than going BONSAI! and bending it too far (each time it's bent, it becomes structurally weaker) and having to bend it back, you might want to let your LBS do this for you. If you are good with tools - have at!
It could also be that the chain needs, or wants to lose, a link or two.