Heater blows cold!

The heater in my car only blows hot air when I’m driving fast. At stop signs it blows cold. You can imagine how this affects my driving strategy. Any thoughts on what the problem might be?

Low coolant level or air trapped in the heater core that needs to be bled out.

bad thermostate low on coolant cj=heck the the simple stuff first

+1. My money is on the T-stat, but it should all be checked out.

A lot would have to do with the age of the car. Just changing the thermostat on a older car and adding antifreeze should be enough to at least test any other problems…clogged radiator/ heater core etc.
I agree too, Big Mark says do the simple things first…but.

thanks all for your help in finding a solutiong for this hot & cold purgatory I’m in. Can anyone tell me where the thermostat is located - is it something I can fix myself, or how much it costs to have someone do it?