Man, I'm snakebit.
Just after I replied to this thread my heater started to act up.
Okay in town, cold on the highway - kinda opposite of what one would expect. But, it was a torn diaphragm in the monovalve - repair kit $45, time = 10 minutes. You don't have to drain any coolant or anything; just move a couple things around to get to the lid, uncrew the 4 screws (don't drop them) and pull it out. push in the bottom plunger and you will instantly know if the rubber is torn.

DG