I recently had a Hyster with a Vortec that started doing the same kind of stuff you were saying. I'm not sure if your truck carries the same vacume governor set up like my hyster. But when i went through this problem theres a electronic controller that regulates the vacume going to the governor. When i checked the voltage it was spiking to 60Volts at times. When i traced the wiring someone had mixed a diagnostic wire with one of the coil wires so it was back feeding to the vacume electronic controller. after fixing the wiring the issue went away.