Temp Gauge swings toward high-side and back to norm with 3 sec

67Dart273 said

I would think if the IVR was failing the fuel gauge would act up as well.

I would try another sender, and most you guys know I don't like "throwing parts" at a problem. If that doesn't fix it, wiring problem, bad gauge unit itself

See if you can see the fuel gauge act up.........at all......it's the IVR

Click to expand...

And the fact that it does it quickly "3 seconds" and goes back to normal means it's not the motor temp varying, but the gauge for whatever reason.
Could be the IVR or sender or it could even be an intermittent grounding of the gauge sender wiring.

Not speaking to you Del, as you know all this already. :D
Just adding to the info and eliminating thoughts of a thermostat or motor heating problem.