You have a fitting on your exhaust manifold that has a water hose attached. That fitting gets clogged cutting the flow of water, could be that. Happened to me once, took it off, it was plugged, cleaned and replaced. Problem solved. Its kind of an elimintation process, I would start with the thermostat as that gets clogged too, really start with the intake to the waterpump and keep going.
Good Luck,
John