air lock would be a good bet, had that happen with a polaris before, somehow an air bubble got into the cooling system, the motor was original, never rebuilt. the solution was to raise the front end of the sled and run it to temp. the bubble would move to the higher end and out the reservoir...
These guys are right about MEC, I have bought from them in the past and found that if you mentioned you were a sledder they would not help you, they would just walk away. I did not know about them supporting closures to sledders though but now that I do the boycott's on!
I am a firm believer...