99 Envoy overheating

My 99 envoy, during the hot days, wants to overheat. When I am on the highway doing 50 and up driving. The temp. Gauge would go up past normal, almost to red then back down to normal, back and forth again. When I get off the highway and stop, it overheats again and stays up. When I turn the truck off for a while, 30 to 1 hr, the temp returns to normal for a while. Because of this, I can not do long driving, during the winter, my temp. Is fine, no problems. If I run the air conditioner, my truck overheats. I have to keep my controls on hot to relieve heat from the engine.I had the thermostat replaced, heater core replaced, water pump replaced, flushed. Can someone tell me what to do.