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?
Since the fuel gauge is not impacted it's probably not the voltage regulator... more likely a shorted temperature sensor or the wiring therein.
Replacing both of the water temp sensors in the housing above the transmission along with the oil pressure switch (I had it and it's right there so why not?) seemed to have solved my erratic temp gauge. Thanks Mr. Waldon!
Love a happy (and cheap) ending !!