Heater Problems Lexus SC400

I have a 97 Lexus SC400 with 178K miles. My heat works fine and blows hot once the car wrams up. However, when I come to a stop,the vents blow cold air into the cabin. Once I start moving again, the heat returns in the vents. I’d love some advice as to what the problem may be. Anything to keep me from the dealership. Thanks, Mark

I would have the water pump checked or look for a vacuum problem. If you have low vacuum due to a leaking vacuum line it can cause the blend door to close when the vacuum goes too low.