Has anyone heard of this happening -
The heat works perfect as long as the car is moving - once you stop at a light or put it in park it blows out cold air.
You have an air pocket in your cooling system, and need to have the air bled out.
What happens is that the the heater core is at a higher level than the engine and radiator, and the coolant level in your system is lower than that of the heater core. When your car is at idle, there isn’t enough pressure created by the water pump to push a steady stream of coolant into the heater core. When you are driving, there is enough pressure to push coolant into it.
Take the car to a shop, and have them properly fill and bleed the cooling system, and you will notice a huge improvement.
BC.
The two best bets are an air pocket as Blade pointed out. It needs to be burped. Depending on your skills and desire, you should be able to do that yourself. The second most likely is a vacuum problem, maybe a loose or disconnected hose.
Note: if that is air in the system, you should start thinking about how it may have gotten there. Leaks or head gasket problems should not be ignored.
I think you may have a vascuum problem with the ducting for the heating. If the car has a small vacuum “tank”, that is what could be causing the problem.