It could be 1 or 2 things : the amp card and/or the servo motor.
to do a quick check on the motor use an ohmeter on the white and brown leads at the amp card - resistance should be 1-3 ohms(approx). if it reads higher or infinity then you have a motor problem.(rebuild or replace motor)
to check the amp card swap the X and Y amp card connections. If the problem moves to Y, then the amp card is bad. If the problem stays in x then the amp card is ok.
also be sure to check the motor connections inside the motor to see there is any moisture/leakage issuses.