Overheating on the Grizzly is a very common thing . The design of the sidepanels and such , doesn't allow the heat to escape enough. Also many people complain about the gas boiling ( mine does it) ... common occurance. A fix could be to use specially designed coolant that helps reduce heat. Another option is to go with the High Lifter radiator kit , which allows the coolant to circulate 3 times before returning to engine rather than once like in the stock radiator. I'll be buying an HL rad this summer.
Also take into effect , that plowing and riding in deep snow is causing the engine to work hard .... therefore creating more heat.
2004 Yamaha Grizzly 660 .... Loaded !!!!
Working Together Towards A Common Goal --- Quitting Time !!
http://www.youtube.com/watch?v=Yx75Zoa_kVw&feature=related