Malibu quits running after 15-20 minutes

1999 Malibu that was running very bad with an awful smell coming from exhaust.Turned out that problem was a bad fuel pressure regulator which I replaced. After that was done the car started right up,idled fine,no smell or smoke from the exhaust and it ran great.However after about 15 minutes or so the car stumbled and then stalled and wouldn't start up.After a couple minutes I got it started but it only ran a couple minutes and stalled again. I parked the car and returned to it a couple hours later and it started right up and I was able to drive it home with no problems only 10 minutes away. The next morning I started the car with no problem and it ran fine while idling in the driveway for about 15 minutes and then it stumbled and then stalled out and it wouldn't start until the engine cooled. Someone has suggested that the oxygen sensor may have gone bad from the engine running rich from the previous problem, but I'm getting no codes.Does anyone have any thoughts on this or any other suggestions.