1999 Chevrolet Blazer • 190,000 miles

My heater blows cold air out the vent most of the time no matter what the setting is on. Every now and then if I keep messing with the dials or if I hit a bump on the road it will start working properly for a few minutes then go back to blowing cold air out of the vents. Can you please tell me how to fix it? I put a new (used) control panel in and it does almost the same thing but not quite. It may be bad as well.
October 5, 2013.

There is a vent door called a blend air door. Based on its position determines if you get heat, AC, or fresh air. It is located near the heater core. Check that to see if it is working.