Two areas to look into:
- Drive the servo without the servo, since it's a one-way out I/O.
<-- this checks the software/coding issues
- Power supply stability
<-- you have to have a quality power supply for Omega2 (and possibly PWM board)
<-- try to use two power supplies (one for logic: Omega2+PCA9685 and the other for GWS S125 servo) to see if the problem would go away