Do you have a smell of sulphur from the exhaust?
I chased an erratic temp indicator around too long, looking at the gauge, sensor, and wiring. The sensor more often went low than high ....
What it turned out to be was a broken cage on the (brass) thermostat: the temp gauge was correctly indicating what was happening with a thermostat that was flopping around. Everytime it popped wide open, the temp dropped .... and the cold start circuit kicked in. This richening of the mixture caused the sulphur smell ..... of the sulphourous acid that ate the belly out of the muffler before i woke up ...
Sometimes the gauges tell the truth - we just need to listen ...