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Need to know what the actual temperature of the coolant is.
You can use the infrared thermometers but I personally prefer a dial meat probe type thermometer placed directly into the coolant at the radiator neck.

When you did the test for exhaust gasses did you use the liquid type and pull air from the radiator through the test liquid to see if it changed colors?

Another way to quickly check for a more sizeable head gasket leak is to remove the fan belt and run the engine and look for bubbles in the coolant.