[Dave at Pelican Parts]

Could it be an air pocket in the cooling system? I've had older cars where that was the cause of that kind of thing.

Can you check to see that the controls are working properly? There should be a valve of some kind to run heated coolant into the heater core. Make sure that it gets commanded to open up when you command heat. If not, the controls are suspect. If it does open, and the core stays cool, suspect an air pocket.

--DD