Running that kind of boost needs an inconnel arm on the wastegate. What has happened is that the turbo has got hot and the arm has twisted so now it is bleeding off a bit of boost. You can adjust the actuator, but you will find that now that has twisted it has weakened it, so it will keep doing so until you run out of adjustment on the actuator.

An inconnel arm is made out of the same material they use in jet engines, so is more than able to take the temps the turbo generates without twisting.

However, I personally think that is WAY too much boost for a T34, and it SHOULD be tailing to 26psi by 7000rpm anyway, or it WILL die (the turbine will shit a blade) .