my 8570 did the same thing I replaced the whole adjusting system solved it. looked at parts when I took them out couldn't see any damage but the teeth must'n been sharp enough. hope this helps
8780xp concave drops
I have an 8780xp with the manual concave adjustment. I am currently combining flax and when I hit a slug or hear the rotor rumble the concave adjustment gear jumps the worm gear and then the concave opens up. I can't seem to get it to stop. Concave clearance is set to 1/8". The worm gear and concave gear are now starting to show wear due to the jumping. Just can't figure out how it jumps as when mated the gears are tight. If anyone has any advice I would appreciate it. Its making harvest frustrating 
