When I swapped my new transmission in I had the opposite problem because I missed the vacuum hose that goes from the turbo plumbing to the wastegate regulator valve. It would overboost on WOT. Once that happened the wastegate would stop opening after that until I shut it off and started the engine again, causing the overboost condition to happen any time it was given throttle at all. Still can't figure out how it worked with that hose missing, but it did until WOT then nothing. Check those vacuum hoses. This valve is attached to the right side of the intake manifold and there are I think three hoses attached to it. They're easy to miss if you've ever had the turbo plumbing off and you cannot see one of them from the top.
