I'm inclined to side with Puke on this one, air in the cooling system.
Try pinching off the heater core bypass and warming the car up, this is the front bottom hose coming off the thermostat outlet housing.
What that does is force water through the heater core and oil cooler, which is were air likes to get trapped.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.