Sounds like you have already tried MBC/EBC.
Check the travel on the wastegate arm. The shaft should rotate through 90 degrees freely with the arm off also. Check for leaks in boost control pipes. Check you are not also running the factory boost controller still trying to control the party.
Also depending on spec, some people run into problems with wastegate capacity, but usually only at high RPM - you didn't say you'd fitted a dustbin sized turbo without a external wastegate