95 Oldsmobile aurora rough idle and hesitates, back fires

I have a 1995 Oldsmobile Aurora and the "service engine soon" light came on at exactly 110,000 miles on the money, we were on our way up north so my husband pulled over and checked everything, everything was fine. He restarted the car and the light was gone. we drove it for the next two days and no problems and then all of the sudden it started acting funny. You start it and leave it in park and it runs slightly rough, the rpms bounce up and down. You put it in drive and it gets worse and jerks back and forth and sometimes make a thump, i think it is back firing... like it wants to take off so you hold your foot on the brake pedal, once you start going the roughness and jerking clears at about 25 mph and then it drives very smooth. So we changed the spark plugs and wires and it is getting worse. What can this be? I wanted to get the codes read but autozone only can read obd2, and this lovely 95 has a obd1 with a obd2 connector so I would have to take it somewhere and money right now is tight.