1998 Elantra

1998 Hyundai Elantra, 1.8 L with around 150,000km
Started off just not being able to start the car once it had warmed up. I replaced the fuel pump, new battery. It got progressivlely worse to where it would just stall. I replaced the computer thinking it was the problem. I also have checked all the wires to and from the fuel pump. The car will start, idle for a few minutes and then shut off. No one including our mechanic seems to be able to diagnose what the next step is. I am desperate for help.