This unit seems to work great most of the time, but will sometimes not move forward or reverse. Example:

Turn key on, move lever to forward position, step on throttle pedal.
Nothing, but if held down, 5-30 seconds later, truck contactor will engage for a split secopnd (jump) and then is once again dead.

Sometimes the lift circuits will not work either.

With some playing and resetting key multiple times, everything begins to work again, and stay working for an hour or so, but then overnight, same issue once again.

Tried to check wiring for loose connections, brake disconnect switch, etc. cant figure out without schematic.
Local Yale dealer is unresponsive to provide or even sell me a service manual. Looking for a service manual PDF please and any suggestions beyond the normal check contactor contacts,. etc.

???