If you experience uncontrolled turbo boosting (too high or too low) and the wastegate chatters as you accelerate, then it is likely that your cycle valve has failed.

A new cycle valve has the following part number:
Q000 3113 V005 0000 00.

In order to do this, you need to remove the rear panel. Click here for coupé removal or here for the standard Roadster.

Follow this guide for freeing the TIK so that you can access the cycle valve.

With the valve accessible, unplug the cable connector and remove both of the pipes. Unscrew the valve from the bracket. You may find that the red/black pipes are very difficult to remove. Therefore, it may be easier to unplug them from the turbo (or wastegate actuator) first.

Refit in reverse ensuring that you connect the pipes the correct way around. See here for correct orientation if you are unsure.