Heater blows warm while driving and cold when stopped

My car is having heating issues when I am driving. While driving, the heater blows and is warm. However, when I stop at a red light the warm air begins blowing cold. The air flow is the same. While idle, the temperature gauge increases to almost the red line. I have checked the fluid levels and they are fine. I have checked the thermostat also and it is functioning. Any thoughts would be appreciated.