Accelleration/Hesitation

I have a question about a 2000 Buick LeSabre with approx. 70,000 miles. The vechicle had the scheduled maintenance done at the dealler w/in the last 2 years. This issue pre-dates the service. The problem is that the car will hesitate on accelleration at a stand still; if you try and compensate for the loss and "floor" the pedal, ussually nothing happens. At other times the car surges quite rapidly. When going up-hill the car losses speed and power and will only go about 20 mph. What's your best diagnosis?