Have a look at the diagram here:
http://media.photobucket.com/image/wastegate%20actuator%20volvo%20850/TIPSP/For%20Sale%20Items/My%20Drawings/Turbo-1.jpg

If the three vacuum lines connecting the Pressure Controller (turbo control valve) are original to your car (1995), there is a good chance that one or more are cracked and leaking. Replacement hose can be had at IPD:
http://www.ipdusa.com/version.asp?strPageHistory=category&numSearchStartRecord=0&strParents=128,372&P_ID=4131&CAT_ID=372&V_ID=10710

... or from here:
http://www.boostcontroller.com/index.php?category=8

Also, if you recently had any work done involving the turbo or pressure regulator requiring the disconnection of the vacuum lines, they may have been reconnected improperly. If you incorrectly connect the two lines leading to the turbo, low boost pressure may result. It happened to me.

If none of that seems to be the problem, have a look at the wastegate actuator to see if has become misadjusted. Look at this web page. Near the bottom is an image of the assembled actuator.
http://www.volvoforums.org.uk/showthread.php?t=48611

Good luck. Let us know how it goes.