Trying to diagnose here when we can't hear or feel it will be very difficult. The mechanic (pref head mechanic as you've had to take it back repeatedly) should take it for a test ride. It's nonsense to avoid that after the 2nd time in, as it saves more shop time and customer grief in the long run. They should also tighten cranks, pedals, chainwheel bolts, etc to spec. Finally have them explain to you the gear combinations to avoid (primarily cross chain and esp small front to small rear).
You could in addition also have a knowledgeable cyclist ride with you to point out the finer points of using and shifting gears.
Last edited by cny-bikeman; 09-13-10 at 08:15 PM.