Define “overheating.” Are you losing any coolant? Is the radiator boiling over, or are you just watching the dashboard gauge? Has anyone measured the ACTUAL temperature of the coolant? The gauge or sender could be faulty.
Are you sure there’s no air in the cooling system? Are the cooling fans working correctly?
Let me get this straight; above 40 fahrenheit there are NO problems whatsoever, regardless of vehicle speed, length of travel, or HVAC setting. Is that correct?