Perform these tests, there seems to be a restriction on the inlet side of the fuel system.
1. Install a vacuum gauge at the fuel service port by the alternator.
2. Run the engine under the following conditions and check the vacuum gauge.
• Cranking - 3 inches to 4 inches HG of vacuum.
• Wide Open Throttle (WOT) in park or neutral - 4 inches to 5 inches HG of vacuum.
• Hard Acceleration driving - 8 inches to 12 inches HG of vacuum.
3. If the vacuum is too high, check for a restriction such as a restricted fuel filter or kinked fuel line.
4. Run a fuel hose into a separate container at different locations on the low (suction) side of the fuel pump.
5. If the vacuum is low, check for air in the system. Install a clear hose or run the suction hose into a separate container to isolate the source of the air.
6. If the vacuum tests OK, check for fuel return from the fuel pressure relief valve mounted in the rear of the driver side common rail during the lack of power mode.
7. Check the fuel return rate of the fuel injectors.
This basically comes down to a fuel line restriction, a clogged filter, a faulty pressure regulator, injector(s), or injection pump. The vacuum testing will turn up a restriction in the fuel system, you've apparently already done the testing for the pressure regulator, so we are narrowing it down.