My Xterra heats up when I pull a grade, and won't cool once I get off the grade- have to pull over let it cool off. Also boils and dumps coolant to puke tank when I shut the engine off, requiring refilling of radiator. I recently have had radiator and thermostat replaced and a coolant hose replaced that blew on most recent 10 day trip to Norcal. 200K on motor - still runs well otherwise. Ideas???