Quite possibly could be the internal regulator cutting out once hot. Not sure if the folks who test it were under no-load conditions?
https://axleaddict.com/auto-repair/A...Regulator-Test
The regulator is only 4 screws though.
https://www.amazon.com/Voltage-Regul...ps%2C88&sr=8-6
https://www.youtube.com/watch?v=XK4SQi88WBc