Another bad voltage regulator?

4 thoughts...
I once swapped a /6 in a 79 D100, motor swung while hanging on cherry picker once broke loose from trans, VC smacked regulator, did something to the connector.... had same problem with it as you describe until I cut the pigtail and replaced it (for the regulator)

Ive had bad batteries cause alternator to go full tilt....

My son has had VR problems on a 72 Fury wagon and an 89 Ramcharger this year, went thru several VRs from parts store.... I went and dug up a couple of new but old Niehoff brand VRs from my stash, probably 40+ years old but brand new.... problem solved on both. good, new, OLD from a reputable brand.

on my 85 D150 I had issue with the regulator too.... wound up going junking and pulling a couple from junkyard refugees (my son used up my stash haha) put a used original VR on and works fine. I have been putting more junkyard parts on my vehicles the last few years and have bene having bette luck getting good parts vs buying Chinesium from the parts stores