this is a known issue with the 4t65E tranny in your car, as well as many other GM models (grand prix, venture/montana/sillouette, regal, etc). This sounds like the valve body needs to be replaced. Is the car shifting hard (1-2 shift or garage shifts)? If so, the PCS (transmission Pressure control solenoid) needs to be replaced. The valve body repair will set you back $600 or so, and the PCS is 300-400 bucks to replace. Doubtful you will need both, but one or the other is required.