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.