My girlfriends car just started doing this:
If she is Idle the AC blows out full blast fine, nice and cold. However if she accelerates at all almost all circulation from 75 vents stop, once she lets up off the pedal it goes back to normal. I tested it with Cruise control also and same thing happens when the car goes to accelerate on its own. Also while driving any time you are "Cruising" allot without acceleration it works fine.
It basically comes down to anytime the car is accelerating the A/C stops blowing out air.
-Branden