Why does my Buick sputter, accelerate on its own and is hard to stop?

Actually Candy the two problems may be connected . You brakes are boosted by a vacuum pump to give you power assist on the brakes and your car sounds like it could possibly have a vacuum leak in the intake manifold which would cause the engine to run like a POS. This can cause erratic idle speeds and backfiring and if the vacuums not there then neither is your power assist on your brakes . The quickest way to find a vacuum leak is to start the car leave it at an idle and take a propane torch ,not lit, but with the gas on and slowly move it around the intake maifold until you notice an increase in the idle speed . Thats were you,ll find the leak .Hope this helps