This thread woke me up in the middle of the night.
I would replace the cot o ring, most likely there is a pin hole leak just enough to let steam out once the cot becomes somewhat dry..

make friends with the neighbor who has the steepest drive way, get the nose up high as possible and massage the firewall heater hose, 2 quarts low IMHO isn't enough to make it overheat..
i had one marauder that the heat stopped at stop lights, it was almost a gallon low, and never over heated..

might try topping it off tomorrow on a steep hill

Regards
Bryan