screwy heat
 

I have a strange problem. I put the dial to red but it still blows cool I have to twist the knob back and forth and then it will start blowing warm air, but even if i turn the knob slightly it starts blowing cool air or I can turn the knob all the way to the cool side but it still stays hot...but mainly it gets stuck on cool. I was twisting that knob for 20mins driving up the parkway yesterday before I got warm air to blow. any ideas what the problem might be and how to fix it? thanks

thank you!