It could be the overboost protection valve. Clean the banjo bolt coming from the intake manifold (I assume you did because of the boost gauge), make sure none of the vacuum lines are blocked from the OBPV. Do the wastegate springs wear out after a while? I would suspect the wastegate for your problem as well, I think you're on the right track.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth