Shall

I have recently purchase an '04 'Subaru Forester, it's been 100+ here most days since I purchased it, I need the AC to work and it does not work properly. When I first fire up the AC to Auto, the fan goes to 4, (high) and I have the temp set on 70 ish. It will blow cold air for a few minutes, then it will start getting warm, it will stay this way and I wind up turning the AC off and putting the windows down. I want my AC! I have tried taking it off Auto and doing it all manually to no avail.... still hot air. I turn it off wait quite a while and then it's cold again for a short time. I have tried leaving it on but turning the fan down and it's just been to terrible to keep it like that..I have tried starting from Manual too and not using the Auto and no luck!!. I am in a sweat box then, the windows must go down. Any suggetions would be very much appreciated.