The most common thing with servos is that they twitch when power is applied. Some are
much worse than others. The GWS servos I have are especially bad.
That's if you aren't even sending pulses to the servo. Then, you have the problem that the
servo will almost never be in the exact position corresponding to the first pulse it gets, and
it will go to the commanded position at maximum speed. So, 2 uncontrollable movements,
(a) one at power up, and (b) the other with the first command pulse.
For (a), try different servos. For (b), you might use some sort of external position feedback,
such as another pot, or alternately, take advantage of the fact that servos have poor torque
when the pulses aren't coming in at a full 50-Hz rate. IOW, send the first few pulses at a slow
update rate so the servo takes a while to get over to its commanded position.