You stated "I might look toward intermittent conditions for the kind of radical jumping ammeter readings that you report. Maybe wiring shorting or bad/loose/intermittent gen and/or VR grounds."
This made reminded me that after i put the first VR on I did drive the tractor down a path in the woods, it was bumpy, when I would hit a bump the VR would in most cases jump to a different level, maybe from -30 to -20, then hit another bump it would jump back up. I even raced the engine while not moving ant the ammeter would go back to +1 for a time until it got jostled some more. Could this be that the field on the generator may be grounding against the generator housing and jostling it causes it to ground at vearious levels? Boy I wish I had access to the trator I would like to try some of these things.