98 Cavalier runs rough
 

98 Chevy Cavalier Rally Sport, 2.2, 2 door
ok...where to start. I just had this car in the garage to get bakess for a sticker. Had the guy hook it to the computer and he got no codes tho the check engine light is on. It shut off after he put it on the computer and ran well for about 10 minutes after i picked it up. The check engine light is back on.
The symptoms are jumping and hesitating on acceleration(worse the faster you try to go from 0) gas smell, stalls often backing out of parking spots. sometimes hard to start in the morning. doesnt stall while going at any higher speeds. I was told the cat has been bored out. would this cause these problems? if the o2 sensor is bad wouldnt a code show up?
Hope someone has an idea. Thanks