'65 302 5-speed MSD-6A 1-wire alternator custom gauges

Starts and runs well. As the engine warms up the gauges start dancing while the voltage flickers downward. It can return to normal for a while, then do it again. Replaced instrument voltage regulator, alternator (with internal voltage regulator), starter solenoid, starter switch, and wire terminals at the voltage regulator. Most of these were pretty old so made sense to replace them while I had 'em out for testing. Checked all the ignition wiring I could get at for possible grounds with a quality v-ohm meter - all looked clean.

Adding electrical load - AC, lights, etc., will often cause it to start. Definitely when the electric radiator cooling fan kicks in, the situation fires up.

Suggestions?

 

Sounds like the voltage regulator but could be older wiring too or a loose ground. Have the alt tested.