The mere FACT that the fan does NOT come on until it is UP to temperature, tells you it is NOT the fan switches sticking on.
Where are you measuring the HEAT at with your IR gun ?
The fans DO take a long time to go off once on if it is a very warm day, (not a problem in the uk!!).
Why not connect up a light so you know when the fans are actually on, the fan switches do go duff sometimes but very unlikely to be three in a row.
I think your heat gun(IR) is out also, so is misleading you, I bet if you leave it long enough it will turn the fans off.
tabetha