I have exactly the same problem however mine is in a rb20det with rb25 highflow turbo. Goin up long hills on the highway if i build boost slowly, at about 5psi boost gauge will bounce up and down (max boost is 10psi atm). But once it it hits 10psi everything is fine. Mine usually only does it between 3000 and 4000rpm and more often in high gears, eg 4 and 5. For the moment i have been putting up with it but would love to find the solution.