[v8volvo]

Turned out I did have a greenbook for instrument cluster and believe it or not, it describes the exact problem you are seeing and what can cause it. Images attached. It sounds like specifically for 1982 through 1984 clusters there is a little built-in voltage regulator unit that is meant to provide a stable 10 volts to the fuel and temp gauges. It says if both gauges are faulty, that is typically the reason for an 82-84 cluster. Guess it probably is intermittently failing, or maybe has a loose connection that you would be able to find.

Section "S22" describes how to test it and to replace it if there is a problem, could be tricky to test if it fails intermittently but given the symptoms maybe you already know enough to incriminate it. Don't know how you'd get a replacement for it now other than from another donor early cluster, pretty sure I have a spare cluster out of an early '84 gasser that might be able to provide one if you or Anders don't find one closer. Or maybe it is even a standard type of electronic part that a Radio Shack or similar could supply a close enough replacement for.

These are from manual TP 31166/2 published 11/88, pages 71-73.