Instrument gremlins.

So it turnout to be the ratty light socket, and that fixed everything.
I started this thread writng about the instrument cluster, and gauges. I found a set of gauges at Summit Racing. Autogage ATM - 2391. They're 1.5" electrical gauges, with sending units. They're just a very basic - temp, volt, oil pressure set. That I think can probably be grafted into the stock dash, without having to worry whether the headlight switch will impede my attemps, and they're cheap. So I can remove the ammeter, and temp gauge that came with the Plymouth, but keep the dash panel. That's good enough for me.