CAR DIES, WONT RESTART UNTIL HOURS LATER

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Bought code scanner and get code P0455. It just passed smog so assume its not gas cap. Have changed fuel filter, also plugs, fuel pump relay, battery. It has died at speeds up to 40 miles per hour. A few times it has died and then started back up on its own (without turning key in ignition) before it came to a complete stop. Sometimes it can be started right back up, and other times it is several hours before it will start back up. Had mechanic come out and look at it and of course it would not die. He cleared code and said we need to get a smoke test, and he didn't have one of those diagnostic tools. Changing gas cap and hoping this cures the problem and can avoid this expensive test.