It doesn't feel like a boost leak, at a certain point it just decides everything is fine and starts pulling like a train, I have also tried to keep an eye on the boost gauge when it happens and it will start building boost then it just holds the revs / boost for a few seconds, then away it goes, a serious boost leak would be evident all the time. One of the possibilities is a fault in the wastgate actuator or its plumbing but I will need a bit of time up my sleeve before I tackle removing the turbo.