Overheating going up inclines is it time for a new thermostat?

I read this long thread and did not see where anyone mentioned that there are two different coolant temperature sensors that can turn on the high coolant temperature warning light. The first is the sensor screwed into the engine. This is the sensor that most people think about. There is a second coolant sensor located in a tee fitting that is underneath the wiper crowl just behind the EGR valve. This sensor is for the coolant that flows through the exhaust heat recovery system. High coolant temperature from either of those sensors can turn on the warning light.

For the OP and Dutchman69 cases, I am guessing that the temperature sensor for the exhaust heat recovery system is the one turning on the warning light. To confirm this, you need a scanner to monitor both temperature sensors. More information can be found in the linked thread. Weird Overheating Issue