You probably need to clean the radiator fins completely and/or the radiator cap is probably bad. I had the same symptoms and a new cap fixed fixed it.
2011 Sportsman 500 HO Touring: Polaris 3500 lb. Winch, Kimpex Adventure XL Trunk, 7" Passenger Foot Risers, UNI Foam Filter
2006 Sportsman 500 X2 EFI: Superwinch Terra 25 w/Synthetic Rope, Kolpin 4 gal Gas Pack, 3" Passenger Foot Risers, 3-Headlight Mod, K&N Filter
SOLD 2010 Sportsman 300: Polaris 2500 lb. Winch, Kolpin Rear Seat (for the dog), UNI Foam Filter
SOLD 2006 Sportsman 700 Twin EFI: 27" Mud-Lite XTR, Superwinch Terra 35 w/Synthetic Rope, Polaris HD Front Springs, OEM Front Bumper Guard, WES Rear Seat, Kimpex Fender Guards w/Footpegs, 3-Headlight Mod, K&N Filter, Symtec Heated Grips