A couple of things come to mind, as your running the engine, whether driving or idling, your getting enough cooling air through the rad, and water movement through the engine to keep the temp down. But, when you shut it down, the heat in the engine will transfer to the water, (as it should) since there is no cooling taking place, (no air over the rad, and no longer water flow) the coolant in the engine area is over heated, and boils. Either you have trapped air in the system, (as has been mentioned earlier,) and/or a rad cap that isn't rated high enough in lbs. pressure before it will releave. As an example, some foreign cars have a electric fan wired up live all the time, when they shut off you may hear the fan come on for a minute or so, and even cycle a couple of times, because the water has absorbed the engine heat and tripped the fan switch. They have the same thing happening as you described, they also have a 8 or 9 lb. cap to help control the temp and overflow. Sniper