its posible the potentiometer in the wervo is going bad... or there may be a partially broken wire leading to the servo. Try a differnt servo in the same location.

Also... if you have a metal clevice on the metal throttle arm you can create RF noise that will drive the controls nuts. I have gone to using ball-links on my throttles to prevent this. (the ball link has a non-conductive pastic snapping on a brass ball. No metal-metal intermittant contact.)