Is there any way the belt is engaging grabbing? I haven't ever seen this happen in 17 years I have been with John Deere. Of course the monitor and bale size tension sensor on the tension arm (which is an input to the monitor) control the size of the bale. I would probably just check the sensor and make sure it is tight and check the belt and pulleys for issues. If nothing is found see if you can borrow a different monitor and see if the issue goes away. Just some ideas, I hope this helps!