The boost is strong most of the time(around 10 psi), but when you go full throttle
at 2nd gear about half the way(~4k rpm) and shift to 3rd, the boost is 0.
No boost at all.
And it comes back to normal after lifting off and wait for a couple of seconds.
If I push it a little harder on 2nd (6k+ rpm) and shift to 3rd, the boost is normal(10 psi).
It has been that way since I bought the car.
Any idea?

Thanks.

 

I had something similar.

Check the 4 vacuum lines under the cross-over pipe that goes from the turbos to the intercooler. Actually, you might as well order some new silicon hoses and replace those 4 to start with.

Let us know if that fixes it.