There's usually a 5k or 10k ohm speed pot (potentiometer) that is used to adjust the speed of the motor, I'd check that first. What brand of controller is it, are the Start/Stop control buttons built into the controller or are they separate? If their not built into the dc drive controller, I'd check them. You could bench test the motor and controler and eliminate any existing electrical conditions. Any mechanical reasons? Good luck.
