No heat or AC in 97 Bravada

I have not found a mechanic that was able to solve this problem for last 18 months. My 97 Bravada blows heat or AC when it is just warming up. After 20 minutes of highway driving the blower does not blow any air. I can hear the blower working as hard as before but there is no air coming out. There is some heat near the air outlets but the air IS NOT coming out. I had blower, resistor and switch already replaced. The worst thing is that mechanics look at it and they acknowledge it when they see it but then they tell me that "it is working fine now". I cannot convey the message that it happens only after you drive fast for extended (20 min) period of time. Lack of competence amazes me, considering $75/hr that Pep Boys charge.
HELP.
MG:crying:

The air HAS to come out somewhere- unless there is a huge blockage- most likely location is the defrost ducts-- Next time when you know it is acting up- feel up to the defroster- is air coming out there- if so, then you have a vacuum leak to the HVAC actuator and/or controller. Does it do it more so.. when accelerating, or climbing hills? If so the most likely scenario is a leaky black plastic vacuum line underhood. These systems default to defrost for safety reasons- if no vacuum is available.....

Does your system have a Fan speed control or do you have the Comfort Control option where you set your desired temperature?

Is the temperature inside the car comfortable?