1994 Ford Crown Victoria Sputtering, backfiring.

My crown vic starts fine--even on cold days. It idles fine. But when I press the gas (even in park) it starts sputtering and even sometimes backfiring. If I place it in drive and try to accelerate, the car just barely wants to move and the engine sputters and sounds like it is either starving or choking or such. If I let off the accelerator, runs fine.

I've checked all fluids and the air filter. Where else should I begin checking? I am not sure if this could be a fuel problem or something else. Thanks.