1.8t BAM Turbo surging

[AceD909]

Hello guys i have turbo surging/fluctuation problem with my 1.8t bam engine.
After i rebuild my k04 023 turbo with new wastegate and new internals.
When i got for WOT my boost goes to 1.4bar and then drops to 0.5bar and goes to 1.4bar again and keeps doing it every second/half-second.
Tried checking for vacuum leaks i didn't find any but changed some hoses that didn't seem good.
Also tried with unplug n75 and the boost is 0.3bar constant and no surging/fluctuation but i didn't try with different one.

Before the turbo rebuild the car was good no boost surging/fluctuation or any other issue the constant boost was 0.7.
Burning oil was the reason for rebuild and it fixed it but i get another problem.

Car is Seat Leon 1M Cupra R BAM 225HP full stock

What could be the issue

 

[Alexis27]

They can be the symptoms of a failed N75.

 

[AceD909]

I tried unplugging the N75, and the boost stayed constant at 0.3 bar. After plugging it back in, I experienced surging again. I ran a diagnostic and got an error code for the N75. After clearing the codes, the surging stopped, but now the boost reaches 1.1 bar at peak, then drops to 0.5 bar and stays there.

However, after driving for a while, when I go full throttle (WOT), the boost initially peaks at 1.1 bar, drops to 0.5 bar, and then on the next WOT attempt, it only reaches 0.3 bar max, until I turn off the car and remove the key.