Wiring prob. Disconnect ALL field wires then use jumper wires with alligator clips to connect batt positive to reg "ign". Jump connect reg "field" to alt field terminal. You're sure the alt/reg are grounded to batt neg properly. See if it behaves now & if so that'll confirm that it is a wiring prob. If not then the alt or the reg (more likely) is it. mechanical or electronic reg? Could have gotten a bad reg. If you confirm it is wiring then replace 1 jumper at a time with its OE lead till you find the culprit. Do the one at a time wiring changes with the eng off. I'm assuming your batt is fully charged and the large cables/batt terminals (positive an neg circuits) are cleaned. Sure sounds like a bad reg, something is full fielding it. EDIT do not go w an internally regulated alt