I'm no expert in electrical matters, but I don't see how adding an additional ground could "fool" the gauge. It's not as though I'm injecting more or less current; I'm just giving what the temp sender does transmit someplace to go. Other members have solved the same problem through the same method or by cleaning the points at which the grounds attach to the car.

I was imprecise when I wrote that the gauge always reads 80-85. The point I meant to convey is that the gauge no longer indicates that the car is running at a higher temperature than it actually is.

In any case, the gauge now accurately reflects what my IR thermometer says through the range of temperatures from 60 up through around 85 and no longer bounces around. Call it what you like, but the problem that existed before no longer existed after I added the extra ground.