If you didn't have the turbo specced with an inconnel arm on the wastegate, then nine times out of ten, it is this that has twisted. What happens is the wastegate arm gets that hot, that the spring tension on the actuator is sufficient to twist it. This has the same effect of backing off the acuator, and thus you lose boost. If it is this, you will have to keep adjusting the actuator to rectify it until you finally run out of adjustment. The only cure is to send the turbo away to have the arm replaced with an inconnel item that can take the heat of high speed running.