acceleration backfire on 92 ,3.1

My wifes car (a 92 grand prix 3.1) started backfiring on take off but smoothed out on the freeway. I thought that she got some bad gas ( in the car lol) and added dry gas. I also changed the plugs and wires. The problem persists. Any Ideas? this problem started overnite.

ok, lets first clarify something here. You say backfire, but a backfire is actually firing out the intake. Much like a distrubutor timing is off, and the spark happens when the intake valve opens. What i think your having is explosions happening out the exhaust. Correct me if im wrong. Possable cause is overfueling. Maybe a sticking fuel injector, or a bad fuel pressure regulator.