Thanks for the suggestions. Control surfaces direction and neutral state were validated by others at the field along with checking for warped wings, out of alignment fuse, etc. No chattering of servos is heard after powering up or returning to neutral, so I doubt this is related to the servos. Next attempt will be with a much forward CG. If that doesn't work, I'll start playing with the thrustline. But that'll have to wait until after the weekend as work and life are intruding on my flying time.