I was here
Try an exorcism?
Sounds a like a reg breaking down under load. Check and clean the main contacts between the reg and the alt (17mm nut and philips head screw from memory)and both main connectors (big and small cable) to the alternator including batt end of main cable and earth cable from batt to eng. If all seems ok. Do a voltage drop test of main cables/connections (while running!)
How's your block ground? If that's good, then yes, an internal refresh should see you good. Might just need brushes.
Happy to have helped. 😉