Ive seen that before. Check your overtravel limit switch. It could be dirty or faulty. Does it improve as the machine warms up? Sometimes as the slides warm up and the machine homes out repeatedly it goes away. Take your finger and depress that little switch with the roller on it and press it like 20 times and see if that helps.
Im just saying that it could be the limit switch, not that it 100% is. Ive seen it happen before.