Originally Posted by Retro Grouch

There are other possibilities, but I think that your local bike shop guy hit upon the most likely one.

Rather than continuing to fumble around aimlessly, goto the Park Tool website and look up rear derailleur adjustment. The place where most inexperienced guys go wrong is to kind of jump into the adjustment process somewhere in the middle. Something is wrong with your bike so assume that the very first step was done wrong until proven otherwise. Then go on from there. My bet is that if you do that, you'll find and fix the problem.