Yes: The AC conderser/CAC/Radiator are clean. I do check on them ever time that I return from a long highway trip. Mainly in the midwest, there are a lot of bugs there at dusk. Garden hose pressure wash and blown from behind with 40 psi compressed air.
Yes: the coolant is rather new. Changed at 5 years and then again at 10 years with the correct one.
Yes a new OEM thermostat when the original failed open, and then another new one ,no fail this time, with the second coolant fluid change, just in case.
The electric fan rotates a two different speeds, but do not make any mechanical noises, just air flow noise.
The mechanical fan have been checked two times, plus I have a new OME spare that I got just in case.
Yes while driving on the Florida flats, AC on full blast, highway speeds, the needle moved to just the center, it is in city driving, stop and go, hard acceleration when the needle move close to the 3/4 mark. I have no had the chance to test the stop and go driving without the AC yet, will do it tomorrow.
Could it be that a fault transmission, shifting in city traffic, cause an engine to over heat, but not the transmission fluid? I checked the transmission fluid and it was at the correct level, correct color and did not felt abnormally hot nor smelled. I am not satisfy with the way that the transmission behaves around 45 to 50 mph at constant speed nor with slight acceleration after some coasting. It is not TC clutch, I am familiar with hat one, looks more as erratic shifting, I am talking with the transmission technician about this, but I can not reproduce the behavior with him on board.
I am hopping that it is not the cylinder head gasket. Timing is not good for that $$$ repair.