The first place I would look is anywhere that could restrict the engine. If you don't know when last changed start with a new air filter and fuel filter. If you feel it is "breaking up" as opposed to "holding back" the ignition system might be breaking down. Now you are on the way to a full tune-up spark plugs, cap, rotor, and ignition wires. Since it is an automatic you should rule out a problem with the shift computer going bad and forcing downshifts. To do this just stick 2 lites onto the diag plug and watch what the shift computer is telling the solenoids. David Teitelbaum vin 10757