Solenoid valve causing power loss?

Hello everyone,

Does anyone know the exact function of a solenoid valve?
above the low-level manifold on a Passat (PD engine, 85 kW, model year 2000).
is attached on the left side with screws?
I suspect that this valve is the cause of the performance loss we've been searching for.
The part has an electrical connector and two hose connections.
It seems to be responsible for the pressure bypass that controls the turbocharger.

Problem: Intermittent power loss; the workshop is, as usual, unable to diagnose the issue.
I can imagine that the device occasionally just outputs its maximum power, which could result in acceleration delays and sometimes even reduce the top speed.

So long .
Thorsten.