We've got a 635F header and a 9660 combine. On the last field of soybeans last year, we started having issues with the head. It would constantly speed up the head unless you held the down button. In that combine, the reel height and head speed is the same button, so this also meant the reel was as low as it would go. Would it be an electrical issue somewhere? An issue with the variable belt drive in the head? It worked fine after switching to the corn head.