The direction of the vent air flow is controlled by a little door in the ductwork. In some cars this door has an electric motor, and in other cars it is operated by engine vacuum. The blend door, which controls the mix of hot air from the heater core, and cold air from the vent or A/C, is also controlled by vacuum. In some cars, the transmission senses engine vacuum, and you have insufficient vacuum, the transmission will think you are under a heavy load and may keep you in a lower gear. Also, if the car thinks you are under heavy load, it may shut off the A/C compressor.
So my 2? diagosis is, you have a vacuum leak.
What year is your Grand Prix and is your Check Engine Light on?