Why sometimes my heater blows cold air unless the car is moving Dodge Intrepid 01 Car has new thermostat and water pump coolant level is correct?

ON the coolant resevoir, look for the hose that runs to a tubular metal connection, it is a smaller hose at the top, make sure coolant flows out when the car is started, willing to bet it is blocked from the point where it connects to the heater core unit. I had the same problem. car works fine now