The boost going away is what gets me, and why I'm leaning so hard toward a problem with the wastegate.
Spark "blow-out" will feel like you're on an MSD limiter, a kind of random misfire, which I'm sure you noticed.
What you describe, sounds to me like a mechanical issue, specifically the wastegate "sticking", either due to binding, or maybe the wastegate control solenoid not functioning correctly.
Maybe try a manual boost controller as a test to see if the problem persists (possible mechanical binding) or goes away indicating a possible computer glitch.
Best of luck with this one, hope it's something simple!
Mike