ORIGINAL: Deadeye
I quit buying Futaba servos after nearly half of my over 30 3004's developed this exact problem. I was told it is caused by a dirty potentiometer, or 'dirty pots'. Hitec servos all the way for me from now on.
Funny thing is I have a NEW high torque Hitec Metal Gear servo on my Yak 26cc gasser with exactly the same problem.
I'm using 6v battery on a pull-pull setup, and the Hitec servo will swing back and forth as it seeks center especially if the engine is not running.
It seems the vibrations of the engine nudges the servo just enough to get it to register center, so with the plane running this tends to stop more quickly.
Ironically cheaper servos ( e.g. HD 1501MG ) have no such problem....
I've had more problems with Hitec servos than any other manufacturer, not to say they are bad...