LMM vs. Wastegate: What's the Difference?

Hello.
I'd like to get an expert opinion on my Volkswagen Golf IV TDI with 90 horsepower, manufactured in 1998, engine code: ALH.
'For some time now, I haven't been able to reach my maximum speed; above 160 km/h, I actually go faster going downhill.'
At this point, one can feel a wave-like power loss occurring at intervals of 1 second.
Acceleration is very sluggish above 2500-3000 RPM.
However, there is hardly any noticeable loss of power in the city, and it offers good torque at low RPMs!
Does that point more towards a faulty lambda sensor or a boost pressure valve?

Thank you very much for your help!
Greetings.
Dirk.

Okay, why don't you check the air mass and boost pressure?

And please read the technical articles on systematic troubleshooting.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.