Don't have any past experience with yours, but could be caused by the
Servo Output IC. Chase the cables that lead from the drive to the main
board, I'm sure the Servo IC will stick out like a sore thumb from
there. Also check in that vicinity for signs of resistors that may have
changed color, most likely not obvious with the naked eye, best to
check them for tolerance. They should be resistors in the circuit
between the IC and the motor itself.