Thanks guys. Its stock and the original VR. How can I confirm internal or external? I've followed the harness from the alternator through the harness apparently to the area under the fusible links and the large black plate where they mount. Is the VR under this plate?
I've got the service manual and section EE18 and reading that I see the adjustment. But, usually, it reads spot on 12.8vdc. So, maybe its ready to crap out and spiking to the pin is its failure mode. I have a large audio load wired directly to the battery and fused properly but its been flawless for several years.
Looking at parts I see alternators go from 75 to 2/77. Mine's a 3/77. Perhaps I have the first month of an internal VR in which case I just need a new alternator.