We have a 110bhp tdi and it suffers from the over boost problem, I had it checked by a garage and they said it could be a number of things, of which all have been covered on here too.
My theory is, if there was a fault with the turbo would it not cut out every time it reaches a certain pressure or drive at a certain amount of revs etc? Instead it seems to do it randomly, sometimes it won't do it for a day or two and other times it does it every time you put your foot down.
Also I noticed it feels like its hessitating/a little flat sometimes, even at low speed which makes me wonder whether it could actually be a sensor or something that controls the turbo?
I've changed the vac pipe from the actuator and that hasn't helped. Could anyone tell me what else to check/try before I start looking for a new turbo.