Shame you are in Syd, I'd be happy to have a look at it...
If it only does it once in a while, try dropping either small hose off the boost modulator (it's a little electric valve to the left of the turbo) and see if that makes it do it all the time, or clears it up, next see if you can join the two small hoses with a bit of metal tubing and see what effect that has, be careful the valve plastic isn't brittle, and the connections break.
I recon it's a Turbo/Wastegate/modulator valve issue.

Fraser