Low power in my AUDI 200 AVANT Quattro Turbo 20V

[Maurizio]

Dear sirs...I will be crazy !
The car was fantastic......when it work fine...but...... 

Engine 3B

1) When it work fine the max pressure of turbo (read on the digital instrument on the cockpit) is 1.7 bar; (in the previous time 1.8 bar).

2) I've change the bypass valve, the waste gate freq. valve, the two knock sensor, all the ose from / to turbo and so on, multifunction temperature sensor, air filter, MAF sensor, fuel pump and fuel filter, the fuel pressure regulator, the number 10 relais....but the operation was very strange!
To control if the overboost send the correct signal to the waste gate freq. valve, I've attached one little lamp in parallel to the connector.
So, the overboost not every time work....without reason work yes...work not.
The max pressure was 1,4...1,5....1,6.....rarely 1,7 when the overboost work fine.
The max pressure was...1,2......!?
The check engine light is on.....without reason..then is off......then is on.....then off (when on for 1 second max)
The only fault code is 4442 ! (In one time 2234)
Waht can I do?
Can you help me...?
Many thanks
Best regards
Maurizio - Como - Italy

I think that I'm the only one owner in Italy of this model, so I want my beautifull car in perfect conditions....the test on traffic lights give me a big smile.....