Have you used a laser gun or some other method of verifying the coolant temperature? Dash gauges can be notoriously inaccurate at times, and localized hot spots can happen that the gauge doesn't pick up right away. Pressurizing the cylinders one at a time with the brakes set and the transmission in gear can pinpoint head gasket leaks and other sources of combustion gases getting into the coolant, such as a cracked cylinder head. Be sure both valves are closed on each cylinder before putting air pressure into the spark plug hole. Look for bubbles in the radiator that indicate a leak from the cylinder to the water jacket.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!