I have a 2000 Chevy Malibu. About a month ago is when I started having problems. It would run fine around town, go anywhere I wanted it to go. Well I had to take about an hour and a half drive, about an hour on the road the car would just die. No warning, nothing. I usually had to wait a while before starting it back up again. Well since then it has become worse and worse. We have replaced the Oxygen sensors, because when the check engine light came on I took it to the auto parts store where they hooked it up to a little handheld machine. It said I needed to replace them. Well, it hasn't helped. It is still doing the same thing, so we then thought the catalitic converter maybe was stopped up, so we took that off and got all the junk out of it, thought it was going to work we drove it with no problem until the next day. Same thing again. It has no distrubutor. There is a spark plug that looks alittle "burnt". We have asked the chevrolet place here they don't know anything. Please help us. I am having a hard time getting my children to school, and to the doctor. What do you think it could be?

Optional Information:
2000 Chevrolet Malibu 6 cylinder

Already Tried:
replaced the oxygen sensors, changed oil, fuel pump, fuel pump fuses.