You are on to something there, with the radiators. Struggled with the hot, AC off bs for a while. I made a number of cooling system improvements at the same time, so it took a while to figure it out. But I think I have it solved on my machine. Source of the problem: Aftermarket radiator, with no flow restriction orifice in the small radiator hose connection, that goes to the surge tank. Coolant was flowing at a good rate from that top fitting into the surge tank, and if you look at the cooling flow circuit, this flow bypasses the thermostat. So it is essentially the same as having your thermostat half open all the time. I put a restriction in the hose, and have not seen the issue since then. Now why the "too smart for it's own good" ECM software is also programmed to interpret this slower than expected engine warm up rate as an overheat condition, is anyone's guess.