Problem with heat / ac control

I am having a problem with the front heat control in my 2001 Chevy Suburban. When I first start the car, I am able to adjust the temperature. After the car runs for a little while, I am no longer able to adjust the temperature. I usually have hot air that comes out of the vents. I replaced the actuator and the control unit and I am still having this problem. Any help would be greatly appreciated.