My 2003 Polaris sportsman 500 is overheating?

It only overheats when its warm out. I use it in the fall for hunting and in the winter for plowing and i have never had a problem. In the summer months, it overheats. The radiator is clean, there is no air in the coolant system, replaced the fan relay switch and tried a new radiator cap. Repair shop said maybe a head gasket, but wouldnt it overheat anytime then, not just during the summer. Suggestions?