basically at high boost if you have a weak actuator spring the force of the boost will push the rod out alot quicker resulting in the wategate opening too early at high revs, but will stay closed longer lower down the rev range which will make the issues you are describing, another possible cause which i have came across in the past was the cam timeing was out so at high revs and as it was quite a fierce cam and it was keeping the exhaust valves open for too long hence loss in power, this was on an rs turbo which only has one camshaft but im sure the same would happen on the twincam YB. just a thought