Rob,
As others have suggested, have a good inspection of J120, J121 and J115 (with game off and unplugged from the wall).
Check your IDC connectors, look for brown or burn marks on them. ALso inspect the header pins on the driver board. Look for any burn marks on the white (turns yellow) or burn marks on the actual driver board.
Finally - in the lower section of the cab, you'll see a loom and plug coming off the transformer (yellow & yellow/white). Again with the game off and unplugged (of course) reseat this plug half a dozen times and see how that goes?
