I agree with all of the suggestions above. One more possibility, there is a limit to how may planes you can reuse your servos, they eventually wear out. I have noticed that metal gear servos wear out much faster than plastic gears, especially if you use a stabilization system. As the servo wears out, it will develop some slop (backlash) in the gears. This slop will cause the servo to become jittery. The test, hold the control surface with your fingers and stop the jitter. Then give the control surface a flick with a finger to restart the jitter.
If you can stop and restart the jitter, you have slop somewhere. It may be backlash in the servo gear, it may be in the linkage or control horns.