c016Y, That is exactly the symptoms only mine was on the elevators. Both sides would react the same way. They would sit an shake up and down all day if you did not stop it. I ended up having the linkage, on the furthest hole away from the servo center point and I believe slightly past that distance on the control horn, with the servo "mounting" screws slightly backed off and that did it. When you mentioned the "retaining" screw on the servo arm, one thing that I experienced was due to the supplied arms being too small and thin I opted for heavy duty arms, Dubro I believe, made for JR servos. Well, they sucked! They didn't come close to fitting snuggly on the armature even with the screw secure. There was play. I ended up getting servo arms right from JR. Have you tried swapping out all your servos to this position and they all do it? The LHS here would not have a problem with letting me try one from stock, providing he has one, to see if it still happens with a new one. There are such things as bad production runs and geography is in your favor as to picking one up from a different run. Good luck, Joe