2006 SAAB 9-5 Loss of turbo boost

I have a '06 saab 95 with 2.3t. With 80000 mls. Its loosing boost pressure when the car runs for 20 - 30 min. Starting cold it runs fine, 15 psi, tires spin, but run it for a little while and the boost pressure drops under 10. It will spike at 12, then fall and hold around 8 psi no matter the load on the engine, the position of the throttle (floored or not), or outside temp. I try to use no less than 88 oct. (91 is the best I can get in salt lake) I use mobile 1 full synth at 5000 intervals with a 10000 mile fram filter. I recently changed the plugs to a nkg e3 and changed the fuel filter, it has a KN cold air intake, also recently cleaned. I have been told that the bosch DV will do this occasionaly but as the car is still under warrenty, im skeptical to change it, especiallly when the dealer tech has never seen one go bad. Any suggestions?