2011 LTC L overheating

Hi everyone, searched some older posts but mostly saw AC related issues. We have a 2011 Executive L sedan with 290K on the clock. Well maintained; runs greats. Used mostly for airport work. No issues except it occasionally overheats. There doesn't appear to be any rhyme or reason to when it decides to overheat. (Unless there is a client in the back and Murphy's Law takes over :cuss:) It can overheat 5 times in a day or go all week with no problems. We've had it to multiple mechanics, each with their own idea of what the problem is. We've replaced radiator fan and module attached to fan shroud, radiator and thermostat. Trouble light shows a P1299 Cylinder Head Temp (limp home mode active). Fan comes on immediately when AC is turned on. I can also hear the fan speed ramp up/down. I don't think it is a mechanical issue but an electrical issue. The only thing I have observed is it seems the fan occasionally stops running as if power were removed. What else can cause the fan to stop? EATC? Could there be a bad ground? Ambient temperature and weather (rain) do not seem to be a factor. I wouldn't think it is the cylinder temp sensor as that is sending an active MIL code. Temp gauge on dash functions as it should. AC usually blows ice cold but starts to warm as the temp gauge rises. Water pump replaced about 6 months ago and coolant level stays where it should in the resevoir; no evidence of coolant in oil. I'm really losing my faith in "mechanics"; I don't want to keep throwing parts at it. Any help is very much appreciated!