Quicksilver said the same thing i was gonna say....radiator cap. But your coolant boiling is a result of pressure loss as the others said. Things to check would the radiator hoses, thermostat housing gasket. Look in the valleys of the lower intake for coolant. If there is some then you're probably loosing pressure. It may be time for LIM gaskets too. I just did them last weekend and my car stopped overheating.