I've had the same problem... after changing coolant. Do as Ryan recommends and make sure all of the air is out of the coolant system.
Here's how I took care of the problem: Start with a cool engine, remove the radiator cap and remove some coolant so the level is down about an inch. Leave the radiator cap loose or off and start the engine. Let the engine warm up so the coolant circulates for a minute or two and the air bubbles should go away. Top off the radiator and you should be good to go.
