@
greg737 has a point. On the other hand, I've purposely idled my 250 in the driveway to check the cooling fan, and found that it does not overheat. The fan cycles on and off, and I watched the temperature gauge needle moving higher and lower repeatedly with that cycle. Was your engine idling at a higher than normal speed at the time as Greg mentioned?
Maybe someone here has experience with the thermostat temperature ratings, but the manual says to replace the thermostat if it opens outside of the range of 146F to 152F.