Well, the SD is in a similar situation.

Starts to run a bit warm.......up near 95°C. or so.......especially in conditions of low airflow. After a highway run and then down to the stoplight runs, the temp will climb to just about 100°C. and sit there. It won't come back until the vehicle gets back on the highway and then it will slowly (30 minutes or more) return to 90°C.

The interesting thing about it is that a brand new OE thermostat performs in the identical manner to the 22 year old unit that was removed. Absolutely no change in performance. The radiator is probably original and the water pump is one year old.

An infrared temperature sensor verified that the outlet temperature to the radiator, measured right at the thermostat housing was 85°C. and the gauge was at 95°C. Checking the temperature of the head also confirmed that it's running at about 85°C. as well. So, the guage is definitely reading 10°C. higher than normal.

However, this vehicle never moved above 90°C. on the gauge unless the ambient temps were well into the '90s. It's sitting at 100°C. on occasion with the ambient temp in the '60s. Quite strange.

I have not pulled and cleaned the radiator and I suspect that it's part of the problem. But, considering that you have a brand new radiator and it's doing the same thing, this might not be the culprit.

I would urge you to check the outlet temperature with an infrared measuring gun and see if the gauge is way off the mark.