Lots of possibilities. A new thermostat doesn't guarantee a good thermostat, nor does any new part. The thermostat could be faulty and restricting flow through the radiator, causing your overheat condition. Similarly, a poor press fit or spot weld on a water pump impeller could do the same. Those would both cause coolant flow issues, which would give you a very large temperature differential across the radiator. More than 20 or 25 degrees Fahrenheit. A failed fan clutch would definitely cause overheating at slow speeds due to lack of airflow. We call that a loss of cooling capacity, and you would expect to see a very small differential across the radiator. Say, less that 5 degrees Fahrenheit. Airflow restrictions through the core would cause the same thing.
You would be best served trying to recreate the problem, and checking the temp differential from the radiator inlet to the outlet with an infrared thermometer. That should put you in the right path.
All that being said, a couple of questions.
1. Did the fan get really loud once the engine was hot and you started moving again? At those temps, it should have locked in tight and roared like a 747 taking off. No noise could mean a failed clutch.
2. What work did you do to the truck recently? You mentioned a new T-stat. Is that all that you replaced?
The last possibility is that your gauge or probe aren't reading correctly and there is no overheating problem. That can be ruled out with the same infrared thermometer.
Let us know what you find.
