When the boost leaks out, the car will run rich, since the air has already been metered by the mass air. It would rev in neutral, because there is little or no boost. Boost should be starting to come on about 3k, so that makes sense.
A guy I know had the exact same problem, and it ended up being the bolts that hold the outlet pipe to the stock turbo. Or rather, the lack of bolts that were supposed to hold the pipe to the turbo.
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Toby Johnson
BlackDog Racing
BlackDog Speed Shop
Lincolnshire, IL