For what it's worth, I just had this exact same thing happen on my '99 diesel sportsman. It would boil the coolant over, fan wasn't turning on, so I figured maybe the thermostat, but what it ended up being was, on mine anyway, there's a flange nut that holds your water pump impeller tight on the shaft that drives it. Mine backed off, and the impeller no longer was spinning when the engine was running, causing the hot coolant to stay in the motor, and the cold coolant staying in the radiator, tricking the fan sensor into thinking all's well... well all was not well, and it damaged the pump shaft in the process, which is also your oil pump shaft and counter balancer all in one, at least on the diesel. That's where I'd look first, as long as you don't have any obstructions in your lines or radiator. Also, try taking you're thermostat out, and either try running it without and see if that fixes it, or just try putting it in a pot of water and start bringing it to a boil and see what temp it opens up at. Good luck!