man, these things can be tough. I agree with wwestman; I had one of their coolant heaters that kept shutting itself down. The problem was the circulation pump wasn't pumping enough fluid through the furnace so it would overheat. Is the HL 32 an air heater? If so, it could be the fan that's blowing air over the heat exchanger is crapping out, or obstructed somehow. How's your air flow?
You have two different air circuits in there, one where air goes in to feed the combustion then is exhausted out the boat, and the other where there is an air intake to blow over the heat exchanger and then out into the cabin. I would take a shop vac, put the hose on the blower side of the vacuum, and try forcing a larger volume of air across the heat exchanger and out into the cabin (NOT through the combustion side). If this works, than you know there's not enough air flowing over your heat exchanger and your cabin supply fan is crapping out.