Just completed my WaterColorBot last night and had the same behavior. Tried RoboPaint and RoboPaintRT on three seperate Macs (different ages and OSs) with the same behavior. Lots of stuttering, some jamming. Just from observation, it looks like it ‘thinks’ it is moving farther onto the paper than it actually is. But if I hit zero, it travels the ‘full’ distance back so it gets to the zero position and keeps trying to spin the motors. Like Sha, I made sure it was in the starting position each time. The only point I deviated from the instructions was that I didn’t unmount and remount the motors – I just wrapped the wires in place. I can provide a video if that would be helpful.