as snowpro mentioned, eliminate the obvious first. also, as unbelievable as it may sound, something as simple as a bad cap loosing pressure. what i have found out on some of the higher mileage sleds, and yours is pushing the threashold, there is a brass gear on the crankshaft that drives the impellor shaft that will spin. it is pressed on between two bearings and since the brass heats faster than the crank, it will work properly for about 10-15 minutes until the metals even out in temp and the gear free spins on the crankshaft, basically not turning the impellor shaft, and the overheat occurs. careful here as the water pump shaft also spins the oil pump.