PC300-5 Komatsu

I have a PC300-5 Komatsu excavator and the RPM's are jumping up and down, the governor motor is shaking and we've looked it over wiring harness and it seems to be okay...also, the power is not shutting off with the key in the OFF position. We've installed a battery switch just to shut it off. Does anyone have any ideas on where to start diagnosing the issue. I also have a PC220-6 with similar problems with governor motor.

PC300-5 Komatsu