Polaris Sportsman 400/500/600/700 3

I have a 2005 Polaris Sportsman 800. Works extremely well in the winter whether riding or plowing snow. As for the other three seasons, after about 2 to 3 hours of trail riding the damn machine overheats. It's like clockwork. Start out fine, run for about 2 hours in all kinds of terrain and then the overheating problem raises it's ugly head. Is this a problem with the Polaris Synthetic Coolant or is it a problem with the thermostat ? I've even tried using additives to the coolant such as "Water Wetter" and it still overheats while my friends leave me in the dust on their Hondas, Yamahas, Suzukis, and Kawasakis. Please send help. I believe this deficiency can be rectified.