535D wastegate stuck

[Esser]

I've been having a problem with the wastegate on a 535d. Sometimes when you drive along, the car won't rev properly, and doesn't pull at all. It doesn't show any warning messages on the dash.
At most times though, it pulls like a train, as it's supposed to
I tried looking under the hood, and noticed that the arm and lever mechanism which is connected to the wastegate, seems to get stuck open sometimes!
When you rev the engine, you can clearly see that the arm moves forward, and then moves back, when you let off the throttle. But, sometimes, the arm moves forward when the engine is reved, and seems to stay there, when you let off the throttle. I think that could be the problem.

Has anyone ever experience anything like this, or know what could possibly be done to fix it?