powering multiple servos from 12v controlled by arduino
If it isn't current limiting causing the servos problems then it's the horrible 0V scheme you've adopted. Try connecting the arduino's supply directly to the servos power feed because, at the moment, you have a very strong possibility of 0V bounce corrupting the control lines fed to those servos. Keep all wires as short as possible.

I see you cannot use the local supply to the servo so just reconnect the 0V as I've hinted.