I had something strange about 5 years ago on my 8770.   Everything worked fine as long as you stayed in 9 or lower.   If you went to 10 which is in the higher range, tractor would stop.   It still wouldn't move if you shifted down, but if you shut off engine it would start in 5 and work fine, just stay under 10.

Had to send the controller to Ag Express and hasn't bothered since.   May not be your problem but those controllers do strange things sometimes.