1997 4x4 z 71 wont start unless its primed ??

97 350 chev truck , it wont start unless i prime it in the trodel body, then runs for 2-3 days then back to same thing again, prime it and it starts . its got a new fule pump, new fule filter , presure is at 50 lbs at idel and runs 60 lbs - 70 lbs when i hit the gas. has any one got any ideas as to why this is happening ?brian says crank possioning sencor is bad and distribotor pickup, and both have been replaced, didnt work??