Car wont start

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Ive owned this car for a year and a half. Whn first bought it ran fine for test drive then after buying it wouldnt start. It started the next day was taken to a shop where they couldnt find an issue, it ran fine. Since then I have had intermitent issues with running good except for slight idle roughness when sitting at traffic lights. Then it started having the same issues as earlier where it would run ok and drive somewhere and tun it off and few min later wouldn't start. Wait a bit go back and it would start right up. Then same thing would happen. We have replaced the fuel pump. Since then it has started several times. Then I went to drive it and it failed to start. Replaced the fuel regulator, still won't start. My mechanic is stumped. So am I. Not sure what to look at next.