fuel pressure drops to zero '92 gmc sierra

[bonenoggin]

1992 gmc sierra 5.7 4wd tbi fuel system
truck wont start when hot sometimes, also dies now and then
it will start back up when cooled off, or at least has so far
fuel filter, plugs, wires, temperature sensor and sender, oxygen sensor have all been replaced. i replaced the fuel pump 6 months ago.
i put a pressure gauge on the fuel line and with ignition on the pump gives me 13 lbs but when the relay shuts the pump off the pressure drops quickly to zero. can the relief valve in the tank cause this?? or is the pump shot already?? or can the regulator in the throttle body be messed up??
HELP!!!