Is it the original Ford Fuel Pressure Reg? It's the rising rate that needs to be right, it needs to be 1:1 so for every 1psi of boost you run the reg needs to increase fuel pressure by 1 psi.
But it produces 5 bar so probably not the reg or a knackered pump i'd go check the wiring next, just put a mulitmeter across the pump terminals at idle and see what the reading is shouldn't be anyless than a 0.5volt drop from the reading across the terminals at the battery.
Rich