Yea I had done the same thing. It just had a full rebuild so I was baffled for ages thinking it was fuel related. I took injectors out and had them tested and they was fine so was then thinking high pressure fuel pump.. I done a compression test and was all ok the only way I was able to diag was, while injectors were out I turned engine over and passenger side bank sounded slightly different. I put my finger in the injector hole losely while someone cranked engine, and on the passenger side you could feel it was blowing but wasn’t sucking finger. Drivers side was doing both. Very vague diag but put me in the right direction that the inlet was opening and closing properly.

Post your results once you diagnose it, will be interesting to see if yours is the same. I have since spoke to couple RR owners and they had exact same problem.

Hi Paul and interested others
In the end mine was a broken cam chain. The engine I was rebuilding had thrown a rod - it took in a mouthful of oil under acceleration/load as it still had the unmodified secondary turbo dump pipe - and when it came to its' sudden stop must have fractured one of the link pins on the cam chain. I didn't pick this up during the rebuild - lesson learnt.
As a side, I didn't want to pull half the engine apart to find out it wasn't the fault so I took some measurements off the broken engine I had, drilled a 1/8" (3mm) hole through the plastic rocker cover above one of the cam lobes, inserted a piece of wire, turned the engine by hand and observed the wire did not move up and down. I filled the hole with some black gasket silicone when fixed.