Make sure you don't have an air lock in your engine. This effect has the air trapped in the engine and the radiator full of water. A t-stat with no bleed hole and/or installed backwards can cause this. I have seen home made gaskets block the bleed hole making the air removal take forever.
Don't run the engine too long because when the steam eventually opens the t-stat and causes red hot cylinder walls to be slammed with cold coolant, things move around fast.