Hey mate
Sounds like your hitting fuel cut mate, its a built in safety feature to the stock ECU to stop you from over boosting and damaging the engine, its not good to keep hitting this cut though.
What spec is the car? I would NOT recommend fitting a fuel cut defender (FCD) until you find out the reason why your hitting that boost level and have the correct fueling in place to make sure its safe to go higher.
If you would like further information and advice on what to do drop me a PM or Email and we can go through it all with you.
Many Thanks
~Nick @ Tuning Developments