Vehicle has covered 100k miles. Lack of power and engine knocking when driving vehicle under load. With scan tool attached when driving vehicle and recording measured rail pressure and rail pressure set point you can see the measured is dropping off confirming a fuel pressure fault. Injector leak off testing initially found 1 Injector leaking off too much, this was replaced but the fault still remains, repeat testing reveals no excessive leak off in accordance with manufacturers spec, a cranking rail pressure test also within spec, supplied fuel pump with a separate diesel supply with known good fuel, there is no regulator valve to the return circuit, there is no low pressure pump, the high pressure pump draws the fuel in, fuel filter history good and no swarf in the filter or fuel tank. So I'm thinking possible faults will be.
1. The fuel pump low pressure draw operation at fault.
2. The fuel pump high pressure operation at fault.
3. One or more injectors are at fault
As stated I've covered the checks for these 3 things, but this is only with the car stationery in accordance with diagnostic methods, clearly one of these is failing when driving the car.
My questions are.
1. Does anyone have any test methods that can be carried out with the vehicle at speed under load?
2. Is it fair to say that if a diesel Injector fails at 100k miles, then it's likely the others will also be failing. I am thinking of getting the injectors tested, I'm just cautious of that as every Injector I get tested is always condemned, in many cases the Injector is not actually the problem?

Hope you can help.

Thanks