I have an f-350 with 6.0 liter diesel Every once in awhile it will not start. It runs fine when running, but you shut it off and it will crank over just fine, but will not fire at all. Sometimes if it sets for 3or4 hours it will start fine sometimes it won't. The batteries,alternator,oil pressure sensor and fuel control module have all been replaced to no avail. This has been going on for almost 2years and the dealer hasn't been able to find anything.

Worn, tired fuel pump?