I had same problem with mine since it sat for a very long time in brackish water. Since it had had no maintenance I replaced all the hoses, the impellor, the radiator cap, and added a large heat exchanger to replace the one that wasn't there. I also flushed the cooling system and that created another problem, the large peices of scale came l oose from inside the engine and plugged the heat exchanger. I had to clean it several times after several trips before I got it all out. It now runs at 160 degrees and is fine. Still has the same thermostat but I probably should replace that. Might be easiest to check the flow from hose where it attaches to the exhaust with the engine running. Should be quite a bit, like a garden hose mostly on. Also check the temperature of the outlet hose from the engine cooling circuit to the heat exchanger, it should get pretty warm after a few minutes. If it doesn't the thermostat is stuck closed or the impellor on the water pump is off inside the engine. Head gasket failure will cause bubbles in radiator or hoses to stiffen with radiator cap on when engine is first started. Good luck.
