Elevator goes up and down ...
One or both elevator halfs and sometimes the rudder go a few degrees up and then a few degrees down. the cycle is about 6 or 10 times per second. If I disconnect the surface from the servo, the servo is rock steady. But with the relatively large surface attached, when the receiver is powered on, it oscillates the surface. If I dampen the surface (touch it with a finger to stabilize it, the oscillation stops). But if I bump it, the oscillation starts again. I think that if there were zero play in the control system, it would be fine. But there is always some play, even with the best system. I am using rocket city links which are pretty good in this regard.