1997 Subrban K1500 4x4 5.7 Litre - Just spent the day replacing the fuel pump (original symptom - car would run for 1 minute - then fuel pressure drop), only to have similar symptom - pressure good - 50 lbs. upon idle out of gear for as long as you want - put it in gear - pressure drops (not immediately - takes a few seconds) and she stalls - pressure around 20 lbs. - then takes a couple of tries and pressure returns and she runs in park - and so on. New parts so far = fuel pump, pickup screen, wire harness, fuel filter. Anyone have any ideas for what to do next? Or what to check? I hate to keep throwing parts at it.