BMW 320d: Power Loss & Turbo Issues

Hello,
'I have a 320d, manufactured in 1998, with 136 horsepower and 165,000 kilometers on the odometer. After purchasing it, the car exhibited a lack of power between 2000-3000 RPM, but this wasn't a smooth drop-off; it occurred in distinct 'steps'.'
After carefully reading the forum and testing with a diode as a replacement for the MAF sensor, I concluded that the MAF sensor was defective. I purchased a new one (which was also new, production code: 11/03).
'No screw marks on any of the screw holes) and installed it. Afterwards, I went to BMW and had it 're-programmed' (whatever that means; I didn't get a clear answer).'
The car still exhibited the same acceleration issues.
Then I installed an LDA (Laser-induced breakdown spectroscopy) and noticed that the boost pressure initially rose to approximately 1.5-1.7 bar, but then suddenly dropped to about 0.9 bar. It was at that exact moment that the leak could be detected. It was also observed that the fuel consumption indicator decreased. Then the pressure increased again (engine speed increased) and power increased. After that, the pressure dropped again. Above 3000 RPM, the boost pressure and power are constant, and the maximum speed is reached.
Since these holes aren't very pleasant, the question is:
What could it be? (The VTG charging adjustment is loose, and the pressure sensor is also working correctly.)

'I may not be familiar with this engine like most people here, but I still want to try and offer some possible solutions.'

How about the VVT (variable valve timing) system? Could the over-revving be caused by a sticking or difficult-to-move VVT? Can you rule out the AGR (exhaust gas recirculation) valve? It might be a good idea to temporarily disable it. What about the turbocharger's air intake path? Is it 100% airtight, or is there perhaps a very small leak? Is the diesel fuel supply okay? Or could the diesel filter be clogged?

Maybe this will help...

Hi,
Not to mention the turbocharger control valve, N75.