97 gmc z71 stalls out

[dggmc]

truck will start fine put new motor tranny and t case in and drove for 400k and then at a light motor died started up again repeatedly and ran fine till i put it in gear then it would die immediatly went back the next day and truck started and ran just fine for about 100k now it dies just sitting there and i dont have to putit ingear to get it to stall now and when it dies you cant even feelit it is so smooth did a fuel pressure check and pre and post start readings are fine but when i shut off engine pressure immediatly drops to30psi then slowwly bleeds down to 18psi in about 1min then stays at 18 any ideas fuel pump and filter replaced in o4 but only 60,000 k on them