Did you put the N75 J valve in the right way round? They go the opposite way to the standard F version. It would make sense, as when it's the wrong way round, the ECU tries to close the wastegate with the n75, but because the n75 is backwards, it opens it instead, causing the turbo to cut its own boost.
However, that would normally make the turbo produce limp mode pressure which is around 600 mBar (9 PSI) iirc.