I have a similar problem. I drive an audi tt 225 quattro. I have put a new ko4 turbo, clutch and even a new 1.8l motor in it. straight pipe, cold air. I've spent a lot of money on it. it doesn't have any vacuum leaks. I just bought an apr r1 diverter valve for it and it didn't fix the problem. stock my car runs 17.5psi, I have it running at 21-23psi it ran hard the first week i got it back after the new motor was put in, but the problem got worse. It hits 21psi holds for 2 seconds and dies out to 10 or 15 psi and sometimes zero then starts building back up to 5-10psi. What the hell is wrong with my car?? I have not replaced the n75 valve yet but this guy on this post has but still has same problem.