we have had the same trouble with a 7700 combine and ran a chopper. we put all new parts and had it all redone and still didnt work. what we ened up doing was making a sheild to cover the belt and that has fixed the problem.
I had trouble with my 8820 two years ago doing the same thing. I put a slow down kit on my chopper and this past fall didn't have one problem with it. I think the chopper is too fast for corn cobs. It beats the dickens out of everything within 50ft of the machine from grain trucks to people standing at the ends of the fields not to mention the cars going down the road in front of some of our fields.