Overheating on road trip

So i took my 85 Supra on a 3 hour road trip the other day and it started overheating towards the end of the trip. It was probably 85 degrees ambient temp. I had the AC on the whole time and around the 2 hour mark I started to see the temperature gauge rise. This was also about the time that the speed limit went from 65 to 75. Never got into the red but definitely started climbing. I flipped on the heater rolled the windows down and the temp gauge went back to normal immediately. Do these cars just not like the high speed or do they not like having the AC on? Do you guys think i should replace the radiator? Recently I had the water pump replaced and the cooling system flushed. Also car recently has had a new head gasket. Ive driven the car around town since with no overheating issues.