It started happening when I installed a used turbo. My old turbo hadn't grenaded yet, but had significant shaft play. It would boost however, and it never cut off, but I'm not certain it boosted as high as this new one.
Could I have mixed up a line re-installing the turbo?
You said to run the vacuum line from the boost source (the red arrows in my pic) to the wastegate, right? That is how that line was run, and I just spliced into it for the MBC.