The Y movement on my Shapeoko2 was being a little jerky on the left motor. I inspected the wires, found a loose one, unscrewed it, had some of the wire end break off, stripped off a new end for it, screwed them back in and now it is moving smoothly again. That motor was moving, but in a pulsing manner. It probably got pulled a little in transit while it was bouncing around in the back of my car. Just a bit of a story in case someone else has jerky/pulsing motion on a motor.