2001 Blazer with weird HVAC problem

My 2001 Blazer has a weird problem with the heating and air conditioner. I can turn on either heat or ac and it will work fine for 8 to 10 minutes then just goes to outside air. My system is manual temp control and vents and fan speeds work fine. Can anyone out there help me out? Does anyone think it may be connected to the vacuum switch on the crank case?