definatly sounds like some sort of exhaust leak that spools the turbo. I"m thinking that your old turbo was not the problem but instead an exhaust leak at the "ram horns" or the feed from the exhaust from the headers/manifold to the piping that goest to the turbo.
I"m still baffeled that we have no waste gate and I don't completely understand how this turbo works compared to a normal turbo? Eitherway, I would get a stethoscope tool and listen around the headers/manifold and all mating surfaces while the truck is running.
Let us know your results