this truck shuts off by itself while in motion

Intermintant stalling conditions can be one of the most difficult issues to repair. Any number of sensors or modules can cause this condition. The ignition module and crankshaft position sensor and their connections would be where I would start looking.

Check your Air Injection System Pump , it may have collected water .

I am having the exact same problem with my 96 GMC 5.7. It will start cold, run perfect for about 10 or 15 min then once it warms up it starts running very bad, Backfires when you give it gas then eventually dies. After letting it set for about 10 to 15 min. It will restart easily. But not until it cools off again. Parts replaced and thrown at it with no sign of improvement: plugs, wires, distributor cap, rotor, cam shaft position sensor, crank shaft position sensor, coil packs, coolent temp sensor. fuel pump, EGR, Ignition control modual. It does not have cats as it is an after market exhaust and cats have been removed. Fuel pressure guage reads 60 psi, What is left?