Maserati Quattroporte p0191 fuel rail pressure issue

Hi
I have a P0191-00 issue that is defeating us! The main dealer's suggested solution was to replace the tank, but fuel is flowing freely to the filter. We have replaced the fuel filter, also drained the tank so don't believe its a dirty fuel issue so failing to see why replacing the tank would solve anything. We have replaced the pressure sensor, the pressure regulator and the voltage regulator. If we cancel the fault code the car will run well for 1 hour or roughly 50 miles. However when we turn the engine off and try and restart it the fault reappears. Has anyone had any similar issues ? what was the solution in your case? happy to pay for professional help if they can explain why their solution will work and give me lasting resolution.