1993 GMC C1500 Have to cycle the "AC" switch to c

Check the vacuum source under the hood, be sure there are no cracks in the line. Where does the air blow when it quits coming out of the vents?

I don't know where the air starts flowing when it stops up. It does not come out in the cab. If I switch to the floor vents or defrost, it will flow better for a bit but eventually, it will stop flowing and I'll just hear the fan motor but no air flow. So I'll cycle the AC button "off" leaving only the vent. Afte 10 mins, the air will flow freely again in 10 mins or so and I'll cycle the AC back on. The air will start flowing cool again and last another 20 mins or so befort the cycle starts over.

I really apreciate the help. Sure would like to figure out the fix for the San Antonio Summers

Does this only happen on A/C ? could be debris against the evaporator core and the fan cant blow through it due to obstructions.Then you shut it off and the debris settles.Inthe HVAC box under the hood, remove the resistor for the fan motor (top of plenum underhood where the A/C lines hook together). Remove resistor and look into the plenum to see if there is alot of debris in there, you can also pull the blower motor out and look into the plenum that way for more debris