Does problem fluxuate to both sides of zero rudder like it's hunting for zero point? If so, usually the rudder feedback in the amp. If it feels like the fluxuation goes to one side of zero then goes back to zero and goes back to the same side as before, check cables at servo for proper torque and tension. If that's ok, check rudder output at amp for equal drive on both sides of zero. If not, replace amp.