As willwren said, the boost bypass is another thing to try, this will determine if the bcs (boost control solenoid) is bad. I had a very similar problem about a half a year ago and mine turned out to be a bad bcs. Only difference was that i my guage would only go up to 2psi. If you need some more help let us know.
While this is happening, you need to watch the boost control actuator on the front of the supercharger, just to the left of the EGR inlet (do this from the side of the car, not in front!)
can you translate this termins--boost control actuator
Remove your supercharger cover. Now look at the picture of my engine in my signature. The Boost Control Actuator is just above and to the right of the oil cap in the front valve cover, mounted to the supercharger. It'* a round actuator with a vacuum line going to it. To the right is the EGR inlet. See my EGR valve with the Pontiac logo on it? Follow the oulet from the EGR up to the Supercharger. Then look to the left for the BCA.