Join Date: Sep 2004
Location: North Central Massachusetts
Bikes: Cannondale R600
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Could be a number of things.
If it's shifting in/out of gears by itself, good chance it's just the cable tension. When you shift, a certain amount of cable is taken in/out. If this doesn't correspond to moving the derailleur to a cog (e.g., it moves it between two cogs each shift), then it's likely to go back and forth. This is even more likely if your bike or cable is new, as the cables stretch a bit with use. Read Sheldon Brown's or the Park Tools blurb on adjusting it.
If you've banged the derailleur, it or the derailleur hanger might be bent.
A worn chain can result in the chain slipping in the same gear.