I would say the governor itself is working properly since you do get overdrive. he top of the governor is removable without taking the whole governor off, it's just 4 screws. You should be able to check the switch that way.
Another thing you might consider, if it all checks out, would be to run a ground wire from the transmission housing to the main chassis ground. I ran one on mine, simply used a length of 10 gauge mounted to one of the governor cap screws, then run to the main grounding point on the frame.
