The heat/air blend function on my 06 Liberty is acting weird. On a cold start, it blows hot air on wind shield. After about 20 minutes of driving I am getting little to no air out if my vents. I can switch to different positions but the air direction does not change. The fan is blowing correctly, I can feel the blower is warm, but it is not coming out of the vents. I let the Jeep sit, say overnight, and start it up in the morning and it is working fine again only to act the same way about 20 minutes later.....please offer any ideas. I am flabbergasted?