Citroën C4 picasso 2,0 CDTi overboost problem

[Pilkasis_]

My Citroën C4 Picasso 2,0 diesel, euro 5, 2011, has a problem. It's turbocharger overboost al the time. Engine doesn't get faults from it, but live parameters shows different story. Turbo fails to stop in time, it stops only with loud hissing noise, to drop excessive boost. I have changed all vacuum system hoses, solenoids (the one that regulates turbocharger - twice), fitted new egr, fap filter, changed two turbochargers- one restored, other garret... Replaced all sensors linked with turbocharger pressure line to no avail... Really don't have any ideas what to do else. Mechanic advices try to reprogram ecu, because maybe someone has messed with it. Second idea is to change turbocharger again. But maybe you, Internet people will have any bright ideas and save me a lot of nerve in process.
Ps. In long distances, error of overboost does show up in diagnostics, but it overboosts all the time...

[xantia_v6]

Check the vacuum in the line to the turbo actuator. If the vacuum is low when overboosting, then the problem is in the turbo. If the vacuum is high then the problem is in the control system.

When you say that it is overboosting all the time, do you mean that the boost is continuously too high, or that the boost is often spiking too high?

[Pilkasis_]

When you say that it is overboosting all the time, do you mean that the boost is continuously too high, or that the boost is often spiking too high?

Turbos boost exceeds the one that ecu requests by big margin, also, when ecu requests shutdown the turbo, it still gives boost. Even while driving and changing gears you can hear rushing excessive air from system.
By "all the time", I mean that graphs for actual boost is higher than requested boost.
Also in bsi there is error "05FF", unexpected re-initiliasation fault coming from...