[mark houghton]

Start with the simple and obvious....Intercooler and turbo seals. You were previously doing a "spirited" and surely totally illegal run up the 'ol rpm scale. Shame on you, to exercise the old girl that way.
Might have dislodged an o'ring or popped a vacuum line somewhere. That can cause serious over rich conditions, as can single pump running and associated WUR power issues.

As Jim already said, it's a wild ass guess at this point, but it seems the common thread when these types of complaints always start with running full on boost and all of a sudden.....crap!