As its only the rudder thats twitching, I assume the others are rock steady and responding, try re-positoning your aerial, again I am assuming a wire closed loop linkage on the rudder that may be transmitting RF interferance.

Mike

EDIT: if your rudder cables cross inside the fus they may be rubbing when the engine is running, put a small non conductive tube over one of the cables to isolate it where they cross and far enough on either side of the crossing to ensure they do not touch when going left right.