Steps you need to take... and us "guessing" isn't one of them.
1. Rule out a bad gauge. Go find a mechanical boost gauge for $30 from Autozone or something. Plug it directly into the manifold. Don't T-off some existing boost line since, for all you know, that line is leaking too.
2. Slipping belt - Sounds like you ruled this out but grease on any of the pulleys is something to look for. Feel your belt for excessive heat. If it's really hot then you have slippage.
3. Tighten all the fittings that you see that may cause a boost leak. Put thread sealer on any fitting that might be suspect.
4. Retorque all bolts that you have access to from the intake manifold up thru to the blower itself. A leak can come from anywhere after the blower and into the heads.
5. Replace all the vacuum lines that you can reach. I once had a cracked vacuum line that you would never have known was cracked until you actually bent the thing.
6. Zip tie all vacuum hose fittings to ensure a proper seal.
7. If you still have a leak, I'm sorry... you'll have to pull off the blower and check the bypass valve. Perhaps there's something causing it to crack open.
8. Once you've got the thing off.. you might as well redo everything from the beginning while replacing/fixing all possible leak sources. You'll need plenty of gasket maker (don't know what they're called), new vacuum hoses, that white pipe thread stuff, etc...
9. After getting everything back together and you're still down 3psi (You should be at 6lbs solid) then time to go shopping for a new blower.
On a side note:
Getting a datalogger like a Zeitronix unit below will save you some time. I've plotted data points during a 2nd gear pull to make it easier to read. My max boost at sea level is a solid 7lbs but this was at 6000ft in the canyons and I rarely get over 5.5-6lbs up there. So a drop of 1.5psi, as someone said, is accurate (not 3lbs). You can see below that this was a 2nd gear to 3rd gear pull. In 2nd gear at 100% throttle, from 5300 rpm to 6700 rpm, boost increased from 5.1 to a consistent 5.5lbs until I shifted to 3rd gear. Then it stays solid again at 5.5lbs.
Yes, I know my AFR is a bit rich and i'm running a tad hot in my IAT but that's a different story.