Mtang78

I have a unique issue that has been happening erratically the past 5 years only in warm weather, not cold weather.

The temp gauge stays in the middle most of the time. But I find that sometimes, when the weather is warm, and running the AC, the temp gauge runs up to 3/4 almost to the end of the box. Car has 343,000 miles on her. I am the original owner. When the gauge does this, I have to shut off the entire vent/AC system so the temp gauge will go back down. Radiator and AC system is all original. I have had different mechanics check this out but they have not been able to replicate the issue until recently, when I had the car at the Ford garage. They were able to replicate the problem. They said the thermostat is getting stuck and to replace the thermostat. But I have my doubts on that being the whole issue.

What I have done in the past couple of years....
Multiple new thermostats (last one in 2018 with water pump)
Radiator flushes
Replaced water pump (2018)
All new step motors for gauges

Thinking maybe we need to replace the radiator and install yet another new thermostat.

Ideas and suggestions on solutions, as well as recommended radiators and thermostats appreciated.