Almost like the wastegate isn't opening enough.. try what ellie says.. could be an issue with the line from the compressor to the wastegate (could it be too restrictive).. Try holding it open and seeing how it goes..
Wastegate is opening.. but not fully.. so it's unable to vent all the excess gas @ a high air flow (6000rpm)..
Check that it is opening fully and easily.. not sure how to test though..