It definitely seems to be "jumpy" but now it doesn't seem to jump as far down as it used to, when it does jump.

I did notice that it jumped a little today when I went over a bump - it was high and then when I went over a bump it dropped down a little. In the past, I observed it jump down 15-20° in a moment.

Also, my fuel gauge is jumpy, but I watched and they don't jump in unison as far as I was able to tell...

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That sounds electrical. I would make sure the connections on the back of the instrument cluster are all tight and maybe try running an extra ground wire from one of the screws on the cluster to a ground point under the dash. These kinds of issues can be a pain to track down.

The infared thermostat is a good idea, just to make sure the engine temperature is correct while you're messing with the instruments.