It happens to all servo's sometime in there life .
possible cause's
1.worn gears
2.faulty trim switch inside the servo
3.narrow deadband
4.wide deadband
some manufacture's try and get the maximum performance out of there servo's like speed and torque and thats when the problem's start to appear.
narrow deadband = high performance but poor efficiancy once the gears wear then you have a major problem with hunting and centering ,sometimes the deadband needs to be set to a low instead of a high on the digital circuit
Wider deadband =lower performance but more efficiency
deadband needs to be perfect for a endless performing servo.