I run a 9400 with and 843 head, needed bigger lift cylinders, anyway it sounds like snapping rollers need replacing. I pulled every other wire out of concave and can run 3.5 to 4.5 mph in corn like you are picking. 220 bu/ acre corn. No trouble feeding.  Could run faster, as far as the head is concerned, but run to much out back of combine if I do.  Do have trouble with individual gear row boxes failing. Lost 2 last year. Have 4 replaced so far. I do not have standard JD snaps. Went with Clark. Maybe they pull the gear boxes to hard.?? Gear boxes are know to have the weld on the tip sprocket fail.