It sounds like you have a number of issues going on at the same time, it's going to take some time to sort through all this so be patient. First of all, I've got to get this wiring sorted out in my head, given the crappy schematics JLG made for this one. I need some help as to which wire is going where. I'd like to make the madness a little more simple by focusing on one thing at a time, other issues will follow. For now, let's forget about high engine while driving and focus on mid range engine speed.
It looks like the red wire on mid range board is going to terminal 14 on terminal strip, that would be one of the ingition terminals. Black wire is going to terminal 18, that is ground terminal. The one I can't tell for sure where it goes is the white wire, looks like it goes to 87A on high engine relay, is that correct? Also, would there be a yellow wire connected to terminal 87 on high engine relay?
The wiring at the Precision Governor and actuator is really vague. Could you confirm the wiring at these components:
Precision Governor terminal 1, yellow wire, to terminal 4 of engine compartment terminal strip
PG term 2, brown, to term1 of strip
PG 3, orn/blk, 13 strip
PG 4, wht/orn, 5 strip
PG 5, wht, 14 strip
throttle actuator:
yellow wire to 4 of terminal strip
wht/grn to 6 of strip
May have more questions, but I'm starting with these.