Well if it wants to shut off not long after it was driven probably when you are under breaking or idle I would try this. You seem to indicate you think it is fuel related. When it dies, put it in park, push the gas pedal all the way to the floor....Will it restart? Will it then go in gear and drive off? ...or does it die. If it dies when you try to drive off I would suspect a vaccum problem. This will not show up on the engines because they cause a problem which overriches the fuel. Then at idle or underbraking it floods out and dies. Kind of like the pump was shut off ...right? If this sound familiar then I would say it is the plenum gasket under the intake. On 5.2cid dodge engines they were prone to failure. Oil gets on them and they disintegrate,fall apart and cause vaccum problems. You can get the gasket kit for like $20 at the local autoparts store. It will take a couple hours to replace. Not hard, but you have to disconnect a few parts in order to replace the gasket. I am willing to bet this is your problem. I took my truck to a bunch of places and they did the regular song and dance, replace plugs, wires, cap etc. Give it a tune up, then let through some sensors at it. Low and behold it was this silly $20 gasket. Again, they are problematic and should be replaced. Mine did all the things your did like clock work for three years and I finally researched it and found the problem. Been running great since. The key was when you said, it runs excellent then all of a sudden. Boy that was familiar. Good luck.