I doubt that you have a problem with the head or cooling system that will respond to any of the normal efforts to correct the problem. I also do not agree with Porkface (with all due respect) in that if you had a bad head there would be other symptoms.

My opinon based on experience with my daughters 88 240 is that that the cooling system just does not have sufficient capacity under extreme heat loads as you described to do the job. I did the thermostat thing, flush thing etc etc, no good result. Her problem was a gradual rise of the guage while stopped at a long light or in slow traffic during hot weather (90F plus) with AC on. I trained her to turn off the AC and flip on the heater to avoid severe overheating and possible damage when this happened.

Final solution was to install an electric fan in front of the rad controlled by a relay and dash switch. She just flips it on when she sees the heat going up and guage soon drops towards normal.

Fortunately she is observant and quite trainable even though she is under 25 yrs of age. What a girl she is!
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David Hunter