Sounds to me that Rick and Greg are on the right path. Not only going out of sinc, but a broken tooth getting caught between the miter gears, stopping operation. Makes sense when you back up, you are dislodging it and it falls from between the miter gears, allowing operation again until next time it finds it way between them.
Assuming this to be the case, I would start with the inspection of the right wheel gear mechanisim, as a broken tooth in the front differential would likely cause both wheels to lock up instead of just the one.