Sh*t Happens, get yourself one ofthose "chainsuck blocks", ( I can't remember what they're really called) that mount to the frame to keep your chin from "flying around". There could be several reasons why you get chain suck that a GOOD mechanic who can SEE your set-up may not be able to diagnose or fix successfully. It coud be something as simple as poor shifting technique, severe angles on the chain from extreme gear positions, a frame design that invites "suck", a burr on a chainring tooth, or the mythical Gremlin.

Ride Suckless
Pat