I have seen 2 reasons for this. The hall effect triggering transistors inside the motor are misfiring. In the second case I had a defective cable that was arcing to the ground wire in the cable. In this second case the machine shook so bad in the Y axis, I thought it would start moving the machine. My meter could not find this using the ohms setting. I cut the cable open to find it. That after replacing everything else. It was a new install.