1997 Explorer Limited Temperature COntrol

My 1997 Explorer Limited will run the heat for 5-10 minutes after you start the car, but if you stop at a light, it goes cold, no matter what the temp is set at. Turning it off and back on, or upping the temp 4-5 degrees will temporarily fix it, until you come to idle again. This is pretty consistent, and happens whether the "auto" is running or it is in manual. However, if you just cruise on the highway, it will work great forever, as long as you do not stop.