Failing Diverter Valve or n75?

[DBXdarkangel]

So yesterday whilst driving back to Devon from Windsor,  the car was driving normal. Then all of a sudden there was no boost around all. Just felt like a 1ltr!
Parked up to do some shopping then came back and it was back to normal. Boosting fine.
Only thing I did notice was when changing gear after it boosted, I can hear a whistle noise like air escaping from a balloon. For a split second.
Had the revision g fitted 6 years ago so do you think it's time to fit a new one?

BTW paid £47 from awesomeGTI and now £62! Ha

[DBXdarkangel]

Just scanned it and came up with this

000564 - Boost Pressure Regulation
               P0234 - 001 - Limit Exceeded (Overboost Condition)
             Freeze Frame:
                    Fault Status: 01100001
                    Fault Priority: 0
                    Fault Frequency: 1
                    Reset counter: 255
                    Mileage: 177328 km
                    Time Indication: 0
                    Date: 2000.00.00
                    Time: 17:36:33

             Freeze Frame:
                    RPM: 2622 /min
                    Load: 72.9 %
                    Speed: 106.0 km/h
                    Temperature: 51.0°C
                    Temperature: 17.0°C
                    Absolute Pres.: 980.0 mbar
                    Voltage: 14.224 V