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Boost pressure issues seem to be the #2 issue in the Volvo Performance world beaten out only by vacuum leaks. Boost pressure control in the 1994-1998 models is a pretty simple system but for the ECU to manage boost correctly it relies heavily on known base values and settings. If these settings are off or if there are mechanical issues then the boost control system can be out of whack.
The primary adjustment is the wastegate setting. Factory setting of the wastegate actuator tended to not only be a quite a bit on the conservative side but also varied considerably. This inconsistency gets compounded when you are running a tuned ECU at higher than stock boost levels. For those running a manual boost controller (MBC) or an electronic boost controller (EBC) this might not be as much of an issue, however even with these aftermarket boost control devices the factory wastegate setting still required adjustment for them to work properly.