ok, here is the situation.
crappy 50 with an OS46, 550T with 8700G. boom mounted servo (with one servo extension), carbon push rod, engine runs smooth, no vibration in hover, new glow plug.
after flying for two or three minutes the tail servo starts twitching, the longer flying the more it twitches. does it as well with gyro gain to 0%, HH and non HH. no twitching with engine off or idle. does twitch at low RPM the same than at high RPM. does twitch in FF the same than in hover. tail belt is tight and not slipping.
did fly with the exact same setup for a couple of tanks and no problems, this started all of the sudden. NONE of the of servos a twitching. frequency check comes out CLEAR.

any ideas where i could start looking... at this point it could be almost anything... any input?

thanks