Hope my experience helps. My 159 2.4JTDM suffered with fluctuating turbo boost, poor performance and juddering when accelerating in 4, 5 and 6th gears from low RPM. Although im still tackling the juddering, the other two problems were attributed to the swirl flaps. The synchro rod had come off leaving the flaps to move wherever they wanted. This caused poor cold starting with lots of white smoke, turbo boost pulsating under hard acceleration and poor performance all round.
I had replaced the intake manifold for new and now the problems are all fixed. I have heard that this is a fault with the design of the manifold and that a mod was released to improve the reliability of the manifold. Does anybody have any comments about this?