1988 GMC SIERRA wont start...tried everything

1988 GMC Sierra 1500 5.7 L.  Tranny went bad, had tranny rebuilt, had to pull motor to put tranny in.  Put new tranny and motor back in.  Started truck and it ran at idle perfectly for 1/2 hour.  Got in truck to move it put foot on accelerator pedal truck died.  Restarted idled fine, put foot on accelerator again and died.  Each time I did this truck began to start rougher and rougher.  Til it got to where it wouldnt start at all.  Now truck wont start.  checked fuel preesure at throttle body reads good within acceptable range, new distributor, new throttle body, traced all electrical lines for pinched wire all look good, put new fuel pump and tank in a year ago, new computer, new coil, throttle position sensor, ignition control module, temp sensor, plugs, and more.  We have replaced everything there is to replace.  Still won't start. Engine is turning over, have spark, fuel, and compression.  Checked timing, checked compression on all cylinders.  Took to 3 mechanics not one have any idea.  Please help!