My first thought is a hole in one of the rubber ducts between the turbo and
the throttle body. Even a hole that your finger fits tightly in will release
a lot of air when the turbo spins up (whether or not the throttle is open
far enough to send boost to the manifold), and that air has already been
measured by the AMM so enough fuel is being injected to use it. The result
is that the mixture is way too rich when the turbo spins. Often it creates a
weird "foldback" sort of response, where everything goes fine even at full
throttle until the engine should start generating power - then the engine
stumbles right down to an idle. If you back off completely on the throttle
it will soon regain its balance until you ask it to put out power again.

Feel along each of those large hoses at each side of the intercooler (you
will probably want to use a paper towel or rag to keep the goo off your
fingers) or remove each hose for a visual inspection. The hole will usually
be surrounded by a soft spot. The hoses are expensive - about $50 to $100
S  - but you would be wise to replace all of them if you find a hole. I did
mine one at a time, but they all failed within less than a 2 year span. I
didn't replace that short blue silicone piece on the turbo, though. In the
meantime it is possible to patch the hose with a cut up aluminum can and
duct tape, provided the hole isn't in a sharp bend.

You will probably also find a hole dissolved through the underside of the
turbo intake duct where it joins the turbo. That duct isn't pressurized, so
it doesn't cause the radical symptoms of the others. Good thing, too - I
paid $150 US for a replacement about five years ago. I understand there are
aftermarket sources with more reasonable prices.

Mike

Do you get heavy smoke from the exhaust when it bogs down?