My truck just dies randomly

So one day Im driving and the truck starts to stumble and then backfires a couple times then the rpms just drop to zero and the truck dies? Pull over and after a couple tries it starts fine. This starts to happen more and more.

Long story short-I take it in for a tune up. New plugs,wires,coil,cap,ignition mod, Fuel system tested(ok). So I drive the truck for a few weeks and then it does the exact same thing again almost in the same spot or area of road? same symtoms.

Take it to the shop and they cant get it to act up? I even go take it out for a drive, around 120 miles over 3days and nothing!

Im desperate at this point to find the problem because Im trying to sell it. This truck has a 502 big block.

First when it dies you need to find if its fuel or spark.
This is what I would do.
Everyone should have these 2 things to narrow down a non running car.
A can of name brand starting fluid with straw. Follow black round tube from air cleaner to the motor and loosen at motor and spray, then try to start. If it starts trouble is fuel related.

To check spark use this between plug wire and plug.
If you are alone pick a plug wire you can see spark from inside cab.

Great, I will try this test. Thanks for the advice.

I had a similar problem with an Oldsmobile years ago. It turned out to be a bad connection on the battery, If your car runs fine, then just stops, I’d first check your electrical connections. Start at the battery, pos/neg hook ups, main ground, then check the connections on you ignition system and connections on an electric fuel pump. A connection could be loose or just dirty. That would explain it just dieing at random. If the electrical system checks out, then check the fuel system.

This is what I meant for checking spark

If the tach drops instantaneously to zero it sounds like your ignition is quitting, especially considering the backfire, which is fuel that didn’t get burned in the cylinders reaching the hot exhaust and exploding there. Could be any part of the ignition system–bad crank or cam sensor (if distributor-less ignition) to a bad coil or ignition module. Intermittent problems are horrible to diagnose-where the vehicle will start right up afterwards and run till the next random failure. I just got done fixing an '03 DeVille DTS with a similar problem. Car would stall suddenly while driving or idling then start again immediately. Turned out to be a bad camshaft position sensor. The computer also stored codes for both the crankshaft position sensors. I was dubious about both of the ckp sensors going bad at the same time but I replaced both anyway after learning that '00-'03 engines of this type originally had ckp sensors that tended to fail after a time due to bad design or manufacture.