rev,
I have the exact same problem, i only have an exhaust, filter and BOV.
I put on a atomic bleed valve to upped the boost to find that it got worse (obveously), then wound it down to find that it was still there. i have have plugs changed, platniums make no diff, and changed the gaps with no luck. my only options left are the Air flow meter (could be that) or the fuel system (like a coil pack or the like). waiting until i get motec before i look further into it. its just too much hassle looking for this thing blind.
Although run BP ultimate, this made it less.
Good luck and let me know what you find out.