Over-voltage conditions???

[Riverstory]

I have now confirmed that I have an electrical gremlin that I've been chasing for a few weeks. So, I'm seeking somebody that has had the EXACT same issue and what fix resolved the problem.

Completely stock 283 with 3 speed, completely stock wiring harness extensively and meticulously cleaned when completing the recent restoration. Original 37A alternator (rebuilt locally by competent shop) externally regulated. 160 miles since restoration.

The panel truck runs famously 99% of the time. Zero issues . . . until it chugged once under power. Thought it might be fuel, replaced inline fuel filter and brass filter in carb. Ran great and I thought issues were gone . . . until yesterday when it did it again.

I "by luck" saw on a digital gauge (not part of my cluster) that my voltage was 17V. I immediately pulled on headlights and my voltage reduced to 13V. And the truck once again ran stellar.

Took the alternator back to my guy who tested and gave green light. Replaced voltage regulator with new when I restored and once again yesterday.

Ran around town today, zero issues until 20 minutes into the ride, I ran up RPMs in second gear (3500) and watched the voltage climb to 16.7 and the chugging began once again. Backed off the RPMs and the voltage dropped to 14V and the truck ran normal. I have ensured that the alt pulley is the correct size.

Battery is 1 year old and is charged periodically with positive cable disconnected. And is in a charged state (12.7V) during these issues.

ANYBODY??
thanks in advance.