has the fuel pressure relief valve on the rail been tested?
i linked a test method for you
note: you cant buy the valve from nissan they want to sell a complete rail you'd have to buy a bocsh on part number 1110010018
it could be an injector but check the fuel pressure relief valve first and then go to the injectors.
there is also a valve on the injector pump that is common to fail on common rail vehicles but seams to be very rare on zd30 crd patrols but to test that you'd need a computer to monitor the sensors to determine if its out of specifications