1994 Pontiac Transport hesitates on accelaration

1994 Pontiac Transport 3800 mileage: 100,000. When you press the accelerator slightly, it hesitates, likes spits and sputters, or when doing 40 mph, when you keep your foot on the pedal slightly, it will shake and hesitate. Does not matter whether it is hot or cold. If you are in traffic, stop, and go, sometimes causes it to stall. (When you accelerate then take your foot off the gas quickly). Any ideas? Changed fuel filter, plugs, air filter, oil and transmission filter. Nothing changed. Your help would be greatly appreciated, I would hate to take it to a dealer and be overcharged without having an idea of what it could be.