[samvear]

Thank you Jason for your input. I am very glad for your help.
I will answer your questions

Have you tried cracking one injector line at a time to see if you can narrow it down to a certain cylinder that isn't firing correctly?

Yes, I did. Result: at the moment of releasing the nut of each nozzle, the engine runs on five cylinders. It starts shaking even harder.
Unable to identify a cylinder that is not performing well.

I'd also run a piece of clear hose for the fuel return from the pump/injectors and see if you're getting any air bubbles.

I did. There is no air in the return pipe.
All injectors were tested on the stand. The high pressure pump is also tuned.