could be the cold weather but it doesnt seem to be surging past 16 in second but 3rd,4th and 5th is pretty bad at high speeds. had a look at the houses related to the wastegate actuator as mentioned by RichX and it looked like the house between the turbo inlet pipe and the ..boost solenoid?(not sure what it is called exactly but it the mechanism that regulates the pressure of the wastegate hose) has been damaged by the heat from the turbo. Would replacing this resolve anything?