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.