Fuel Pump problems of the 2006 Buick Lucerne

I own a 2006 Buick Lucerne. The vehicle slows as if it is going to stall and cut off, it picks up speed on its own ,and when stopping the vehicle cuts off. It presents a safety hazard in that most of my traveling is done via the expressway. When the car slows down I am at the mercy of the driver behind me. If he is going too fast and tail gates then I am sure to be rear-ended. I have taken this car to the dealership five times,and they could not duplicate the problem. Diagnostics showed nothing. Service department manager drove vehicle while in shop for fifth time and experienced the problem. Because diagnostics still doesn't recognize a problem he says they are having to call in an expert to determine the problem. The car has been in and out of shop since April 3, 2007, and each time the engineers from gm supposedly were in consultation with service department. This problem occurs randomly with no warning. Help!.