Problem solved !
Though I am not sure which move corrected the gauge jumping issue.

Grounding strap from body to frame on passenger side was missing - replaced it.
Grounding strap from block to firewall was at some point removed and replaced with simple wire that was of too small a gauge - replaced it with the correct grounding strap.
Battery terminals were worn, with negative side ever so slightly deformed that tightening fully was impossible. Didn't appreciate this until I applied some pressure to it.
Replaced both terminals.

Now turn on the fan, and voltage gauge ticks down 1-2 volts (just at switch throw) and comes immediately back to around 14 volts. Temp and oil pressure never move.

Thank you for all the advice.
I did pick up a relay and will keep it on hand. That was a great thought. It will be put to use.