[THEFERMANATOR]

Like spare suggested, check for compression entering the cooling system. If your losing coolant under load, you may very well be losing compression to the cooling system. Get somebody else to go with you on a run and remove the pressure cap and put it under load. If bubbles appear your losing compression to the cooling system. If you don't see bubbles, then I would disconnect your raw water line from the gimble at the heat exchanger. Then start the engine and see if you have good water flow. Then disconnect your outlet from the heat exchanger to the exhaust elbow and check there. You may also try putting a pressure guage inline after the heat exchanger but before the exhaust elbow and watch the pressure when your problems occur. This will tell you if you have water flowing, or if you have a major obstruction.