I ran some temp vs resistance tests for the sender. It's a thermistor that changes resistance with temp having less resistance as it heats up. It seems to function that way although I can't verify if the numbers are correct as I have no specs on it.
I did the Bentley test, grounding the temp gauge wire direct to ground, bypassing the thermistor; that is, simulating a high temp (low resistance) condition. My gauge read only about 3/8 full deflection when it should have read full deflection for a simulated high temp condition. To me, that says the gauge is faulty. But this is electrical stuff and it always baffles me or melts something.
I think there is a small solid state "voltage stabilizer" that converts 12 volt to 10 volts and feeds the temp and fuel gauges. I read only 8.6 volts at the sender until I jiggled the wires and the #16 fuse that protects that circuit. Volts jumped to 9.6 volts. That helped a little with the temp gauge and got it to read about 1/2 full deflection at operating temp.
So I'm still stuck where I started. Some time ago I installed an auxiliary temp gauge. I screwed its sender into a tapped boss on the front of the engine block. Its readout correlates very closely with a temp gun reading of the radiator outlet flange on the head. So I have a reliable temp read out.
I think I will replace the sender and see what happens. I have no solid reason to do that and expect different results, but, as I said, electrical is not my strong suit and sometime magic happens or smokes!!!!!