Is there anyway you could post a part#. I'm no expert on these engines, but I know for fact that they do not have a wastegate. The VGT vanes control the amount of exhaust backpressure inside the turbo by opening or closing. So I'm curious as to what part you had replaced/problems with. It could be someone is using the wrong term for a part. The price and problems you gave sounds like you had the EGR valve replaced.
It has what is called ERTV. Something with electronically controlled turbo vanes. It also had the wastegate valve on it like I mentioned in my post. If you look at the turbo from the passenger side, it is right in the middle of the turbo between the exhaust and the intake wheels.
Based on that description, I think your referring to the VGT control valve. From what I understand this valve uses oil to operate the piston that controls the turbo's vanes. Either opening or closing them. So I guess in a very roundabout way it does the same job that a wastegate would do, but it's no waste gate. Unfortunately someone with actual experience working on these engines would have to tell you why it's being a problem.
You are certainly describing the VGT actuator. If you've had to have yours replaced several times, perhaps you've got another problem... probably stuck or sticking vanes in the turbo. It doesn't function as a wastegate, what it does is open and close vanes in the turbo to change the size of the turbine housing to give you the best all around performance.