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

It happens once in a blue moon, but not every drive. It might even happen when I am not even looking. I had a 1971 Dodge Demon 318 back in 1989-1994 that would the same thing in the summer, but not doing winter months. I have seen it happen twice this summer. I am running a Mildon 180 high flow thermostat, but last year it happened with a stock style 195 in the 340 as well. It happens after driving for 20+ min.

My fuel gauge does not change position. I recently changed to the thermostat to the Mildon 180 high-flow. The 360 had a previously had the same 195* degree stock style as the old 340.

1973 Dodge Dart Sport H code