Unlike most of the temp gauge problems I found while searching mine does not involve the fuel gauge. I was driving my 1981 caddy 1.6n/a and the temp gauge started acting up, up and down randomly from indicating stone cold to pegged overheating in seconds sometimes stopping in the middle for a few seconds.
Now as soon as there is power ie: first key position, the temp gauge pegs all the way to the right quickly and stays there.
Could this still be the voltage regulator? Grounding issue?