Did you confirm that the thermostate is working?
I had a very similar problem. Ok at low speed. Ok at idle. Temp climbed at higher speed. I had had problems with overheating in the past and just assumed the problem was some deficent in the cooling system. I replaced the stamped impeller water pump with a cast iron impeller pump, replaced the lower raditor hose with a molded hose and added a internal support spring, clearanced the water pump divider and it still overheated. (Had previously replaced the fan clutch the year before trying to reduce the operating temp.)
Changed the thermostat. Problem solved. red-faced, but now I feel pretty good about the condition of the cooling system.
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