I have a 2003 chevy 1500 hd 4x4 crew cab. For over a year now when you first start the truck the air blows fine. After about 5-10 minutes of driving the air slowly stops blowing from the vents and starts blowing into the middle of the dash. It is not coming out of anywhere. Not the defrost, floor or face vents. You can hear it but cannot feel it. It does the same thing with heat and air. I have replaced the ac climate control, the mode actuator and have reset it in every way I have read online to no avail. I really don't know where to go from here. i would greatly appreciate any help / advise you may have. Thanks in advance!