1991 Chevy Corsica

Engine Performance problem
1991 Chevy Corsica 4 cyl Automatic 143000 miles

I just bought a 91 Corsica for 600.00. Thought I was getting a very good deal. This car has sat in someones back yard for about a year. It was running fine the first day I got it. The next day when I cranked it everything went fine. Then as I drove it, it seems to have lost power. Sometimes when I start to pull off it will almost die. Or when im going down the road and its doing fine I push the gas a lil harder to pick up speed and it jerks and jumps, so I let off and softly press it and it does fine. Well it got really bad a few days later. Every time it was just in idle it would die. Or i'd go to drive or reverse and itd die. Or if I finally got out on the road it would just jerk and jump. We changed the plugs and plug wires. And filled the tank up with gas. ( I had also filled it up the day I bought it.) And I put fuel treatment in it. What could be wrong? Or what else could I fix?