servos get a position command and then they move the servo to the commanded position (comes from receiver). when the command doesnt change its still trying to hold the last position commanded. in the servo is a program that someone wrote that tells it to use for example 1 amp of current to correct the difference of position from the command and what it reads on the internal feedback (pot).

the guy who wrote the program had to make some educated guesses about the mechanical assembly and that is when he decided 1 amp of current is the right amount to bump the motor with to make small corrections.

2 things go wrong commonly.

1-the friction and mass that the programmer made his guess from has changed. so 1 amp actually over corrects the difference in command versus actual. so it overshoots the correction move and then it overshoots again and gain. this is an oscillation from a bad correction move. increase friction and it will stop.

2-pots wear out or experience noise. the receiver sends a command and the servo moves. it watches the pot to confirm it made it to the commanded position. if the pot doesnt read the expected value it will try to correct it until it does. if the pot has a bad spot or their is noise on the pot the voltage from the pot fluctuates and the purpose of the servo is to stabilize the position (the fluctuating voltage). if the voltage isnt stable the servo will move to stabilize it.