another rebuilder won't be able to duplicate the "heat". If a rebuilder rebuilds your maybe crazy alt you may have the same problem return, don't know. Most likely guess is the regulator.

maybe there is a wiring prob somehere, even a bad ignition sw.

Or have a buddy follow you here if you need a confirm? Best gamble is to buy a new regulator.. Good luck.
fwiw, I installed a volt meter.

 

I wonder if you could accelerate the test by using a hair dryer/heat gun towarm various components of the alt. on the bench? Also check the resistance across the brushes; it should be near zero. On my alt, max brush voltage is about 4V. at that low voltage, even a small increase in resistance will dramatically drop output voltage. The heck of it is, it is easier to find these things when they don't wrk at all. It is time to check the details and play with heat to locate the source of the change with temp.