[Ninjabread]

Well, could be an air pocket in your coolant system. Did you guys bleed it properly?

Worse case scenario is your head gasket. But look for any leaks, and try bleeding it again.

The easiest way is to drive the front of the car up on blocks, or jack it up to have the motor at a backward angle. Make sure you turn the heat in your car all the way up. Make sure your overflow tank has some coolant in it. Now, open rad cap, start the car, wait until it heats up and the coolant starts to flow. At some point it will start going up, so put your rad cap on and let it run for another 5 minutes.

Turn it off and let it cool down for 15 minutes. Open the rad cap, top off the fluid and repeat.