2003 Ford Focus doesn't always start

Sometimes my 2003 Focus will start but not stay running. It only happens after I’ve killed it for a short time, like running into a store and out. If I let it sit without trying to start it for 20-30 min then it will start and be fine. Sometimes it will do this everyday for several days and then it won’t do it for weeks or months. I have had the fuel filter replaced. Then I took it in and they did the diagnostic and no codes came up in history. Other than this one thing I like the car, but this alone is about to push me to getting rid of it. Any ideas?

Leaking fuel injector?
(Does it seem flooded when it won’t start or starts and stalls? Any black smoke or gas smell?)

You need to determine if the trouble is due to a lack of fuel or lack of spark to the plugs. You could have a can of starter fluid on hand when the trouble occurs again. Then spray a small amount into the intake and see if the engine tries to start then. If it does there seems to be a fuel delivery problem. Possibly a intermittent fuel pump relay. If the engine still makes no effort to run then the ignition system needs to be checked out for a problem.

Try a few simple things. When it doesn’t start, use the “clear a flooded engine” procedure: Depress the gas pedal to the floor, and HOLD it there, while turning the key and cranking the engine for 10 to 15 seconds. Let the starter rest a minuet, and repeat. Ease up on the gas pedal when (if) the engine starts.
Not ENOUGH fuel: turn the ignition key ON for two seconds, off for two seconds, on and off. Attempt to start.
Notify your mechanic of the results. Also, somebody is here 24/7.