FYI, EVC is a model, EBC is the type of device: Electronic Boost Controller... May prevent confusion to refer to it as an EBC or give the full name of the device.
The flutter when shifting is compressor surge; It's indicative of a bad BOV/BPV or (if it's an aftermarket and if adjustable) an improperly tuned BOV/BPV. Unless your wastegate is frozen in the open position everything sounds more like a bad BOV/BPV. You may have blown an intake coupler too.
Typically when the wastegate goes it fails to open causing you to build more and more boost as the engine load increases. You would have hit up to 12psi before you hit fuel cut.
Remove the EBC from the wastegate's vacuum line until you have this sorted out, then put it back and get it retuned.
Check for diagnostic codes too, you've probably got codes for airflow or abnormal turbo pressure stored; See the MKIII FAQ section for info on how to do this.
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