Wow,
This brings back memories.
I chased a problem like this in the 80s'.
replaced just about everything electrical that I could bolt on.

Turned out to be a damaged-broken ammeter gauge. when the loose post on the gauge was in the right place, everything was fine, when it moved. "GHOST" car with strange events.....
Climb under the dash and check the ammeter wires for tightness/broken posts.