Truck starts and runs great but will suddenly die while driving.

Sometimes it runs great, for weeks at a time even, but sometimes it won't go more than a few miles before it dies without any warning, like it isn't getting any fuel. Always fires right back up. Usually happens when fuel is low, but has happened when full. Had codes pulled, fuel mixture is rich. We have replaced the fuel filter and checked all relays and fuses, fuel lines, vacuum lines, fuel pump does come on and fuel pressure is good. Any suggestions?