My bet is that the little rubber boot on your fuel shutoff solenoid has deteriorated and is jamming up the movement of the shutoff rod. That solenoid has two coils ... a big "Start" coil that is energized when you hit the starter. Once you release the starter, the solenoid drops to a second smaller "run" coil. With the rod boot jamming up the works, the start coil will yank open the fuel valve but the run coil isn't strong enough to hold it open so the fuel is cut off and the engine dies. That's what happened on my '97 Ram 2500 CTD. Geno's sells just the boot at https://www.genosgarage.com/product/sa-4153/fuel-system-accessories-replacement-parts