The problem in my temperature and fuel gauge (previously described) was caused by an intermittent short in the ground behind the instrument cluster. A competent mechanic identified this and wired around the ground, solving the problem and also making other features of the car work again that I didn't even know existed, namely the seat belt warning buzzer.

Note that before I owned this car (more than 10 years ago) someone had stripped the recessed bolt holding down the steering wheel. This combined with what was presenting itself as a complicated troubleshooting process had me give the car to the mechanic. How the mechanic got that bolt off of there is a mystery to me. He did it and installed a new one.

So my answer is that, while logically it would appear the gauge is going bad, it may be a bad ground. For further study is a task, for me, to determine how this intermittent ground has developed in the wiring harness. My major suspects are an alarm system installed in the car 10 years ago (I've deinstalled the "brain" but it's still hanging in the harness) as well as looking for any place where there may have been a short and melting, or anything loose, etc. This is a long-term look for me. At this point everything looks pretty good. The only thing semi-off at the moment is the hazard lights don't always come-on at the first hit of the switch, usually takes two its. I don't htink it's the switch, it was never a problem before. I believe it's related to the intermittent ground.

Note after this fix, the car reads a correct temperature and the fuel gauge is correct to within an about 10%