A few things...
1) The DIC boost gauge isn't very accurate...unless you like telling your buddy's your boosting at 8 lego's not 6. Get a cheap-o boost gauge and tap it into the vacuum line behind the blower under the MAP sensor..."T" it in. That way it'll tell you your real boost levels AND if you have a vacuum leak or not. It should sit about -20 vac at idle...and probably make about 5-7psi at WOT right before you shift.
2) Check for vacuum leaks...if it has sat for a long time it most likely has suffered from the typical 3800 dry rotted vacuum lines...there are 2 sets...one from the Throttle Body to the EVAP solinoid on the top of the back of the blower...2nd one is the full set from the BBV (Boost Bypass Valve), to the FPR (Fuel Pressure Regulator), to the LIM under the blower snout (Lower Intake Manifold), around the back to the bottom of the MAP sensor where your "T"-ing in the boost gauge too.
3) A good way to check for leaks is when the engine is warm and idling...spray some brake clean around the area where the hiss is coming from...it will suck in the brake clean and change your idle...and possible make the car want to die...then you know you've found your leak.
Hope this helps.
