My Truck cranks but does not start

My truck cranks but will not start.  It will start and run a few seconds on
starter spray.  I swapped the fuel pump relay with my truck that runs and the
fuel pump driver module.  I tested fuel pressure at the rail and I have 64psi at
key on and 52 psi at crank.  The time the truck started after I sprayed starter
fluid in the intake it ran but the fuel pressure dropped right off below 15 and
the truck would stall.  I checked all fusses.  I unplugged my mass air flow
sensor still no start.  After the truck stalls I still have 52 PSI at the rail as soon
as I turn the key to on position but won't start.  My pats lights on the dash
check out.  They flash slow while key is out.  They stay on for 3 seconds
when key is put in and then stay off while trying to start.  I also checked
spark at a plug and I did have spark.
I'm not sure what to check next!
any help much appreciated.