2000 Oldsmobile Silhouette hesitating

Engine Performance problem
2000 Oldsmobile Silhouette 4 cyl Automatic 141000 miles

My vehicle hesitates when I step on the gas. It feels like it is going to stall, the RPM's jerk all over the place, sometimes it backfires, the change engine oil, sensor tracking etc lights pop on and off. If I let up on the gas sometimes its ok - other times it just gets worse. I usually have to pull over shut the car off and then restart it. There is no rhyme or reason as to why it is doing this or when it will do it. I have replaced the catalitic converter and plugs, wires, pvc valve - no one has the answer. Has anyone else had this problem? Seems to be fine the first 15 minutes of driving then acts up. I need help!