I just got out of the same problem with my 81SD. The gauges began jumping around, and the temp gauge would climb and then flick back down. It started slowly last spring.
I bought a can of electrical contact cleaner and started searching for a bad connection. Went to the instrument cluster first. Traced the harness to a buss in front of the tach amp, mounted on the inside of the left front fender. That was the problem. It has some large pins and the connection between the male/female had corroded a bit. Some electical contact cleaner and some electical grease to protect and now the gauges work again.
Be carefull about the position the buss plugs in. Easy to get it backwards. Then the gauges will freeze in one position. Just unplug and reverse.
Took a while, but I didn't spend that much time on it. Easy fix.