1998 Dodge Caravan Occasionally won't start

My van for some reason, about every 2 weeks, decides it doesn't want to start. If I leave it several hours, it'll crank just like nothing wrong. This happens when it's cold or hot, doesn't matter. I've jumped the starter cylanoid, and it turns over but will not crank. Like it's not getting fire coil. I've replace the ignition switch, start nuetral switch, tested the fuse link off the main fuse panel under the hood, changed all the relays. Mechanic told me it could be the alarm (anti-theft), but this van does not have an alarm system in it that I know of. There are no lights on the dash about one anyways.