The BPC (Boost Performance Control) valve is located on top of the fan shroud, on the driver's side (US). It is secured by two Torx screws (T25, I think). It is also sometimes called the APC (Automatic Performance Control) valve. It also maybe called a solenoid.
The device is cylindrical, a bit over an inch in diameter and around 2 inches tall. It has an electrical connector (three contacts) and three vacuum connections. It is a common problem for these devices to stick, causing reduced boost (typically) or even over-boost conditions. When they act up, they can often be fixed by simply spraying some penetrant/lubricant like Liquid Wrench in the vacuum ports and blowing or slinging out the excess. This works some of the time, and it's certainly worth a try. If boost performance returns to normal after such a treatment, you know the valve/solenoid was the problem. If the low boost or overboost symptom reappears after only a short time, then the valve will probably have to be replaced.
Since it is a $210 part from Saab, which can be purchased for less from a dealer that discounts (e.g., Kachina in Phoenix at 800-955-1205), trying a used one for as little as $30 or $40 may be worthwhile. English-Swedish (800-241-1916) and Goldwing (800-722-2137) have good reputations and will guarantee a part that works when received. Discuss the warranty before the sale to make sure you understand the limitations .