1998 isuzu npr wont start

i have a 1998 isuzu npr wit a 350 motor i tried everything to get it to start and it will not start. i put a new coil, new ignition module, new pick up ,new crank sensor, new computer, new fuel pump, and every once in a while the fuse relay center right behind the driver side wheel ill turn the ignition on the relays start clicking away then it will start and it will shut right back off and then nothing no spark or fuel any advice