When I am riding, the two pulleys on the derailleur "arm" are getting pulled forward then released in a snap (the arm is swinging back and forth as I pedal). This is happening intermittently while riding, not while off bike, and it happens on flat ground (I haven't tried it on hills).
The bike is new, and in an attempt to solve the problem, the LBS has changed the chain and rear derailleur, but the problem still persists. They also checked for things that would cause chain suck, but the cranks are new so there is none of the normal wear that would cause chain suck.
What else could the problem be?