The "threshhold" at which the problem occurs, does it occur right around the time that the boost needle is pointing straight up (zero manifold pressure)? If so, you may have a cracked hose somewhere between the turbo and the throttle body. A leak will cause the car to run lean at idle (high rpm), rich under boost, and ok in the middle. It would also explain your whistling sound. Pay close attention to the hose between turbo and intercooler, it sees a lot of oil. Good luck.