2000 Chevy Malibu 2000 Malibu dies every 20 miles

My chevy malibu was a great car until now, a couple weeks ago I ran out of gas, and had to put in gas from a old container which was a little dirty, and ever since then my car will die ever 10 or twenty miles or so, and I also live in a very hilly terrain. I took it to the shop thinking it needed a new fuel pump, but that was not it, they hooked it up to their computer and said it was a bad o2 censor, so I got it back yesterday and it is still having the same problems. And I put in some new transmission fluid thinking that might be the trick since it was a little low, but it wasnt. Can anybody help me out on this?