I'm surprised noone has mentioned the Air Recirculation Valve (ARV). The ARV modulates turbo boost under the control of the engine ECU. The ARV is distinct from the wastegate, which is completely mechanical.

We don't fully understand how/why the ARV was used. It seems to have something to do with emissions control, in conjunction with the dreaded trap oxidiser. If the trapox has been replaced (it has, right?) the ARV is not necessary (foreign market cars without a trapox also did not have an ARV.)

The ARV can be disconnected. It is controlled via one of the two vacuum valves beneath the air cleaner. Simply disconnect and plug the vacuum line to the ARV. See if the car drives better.

On an '87 I used to own the ECU would more or less randomly command the ARV to dump all boost. Just cruising along the car would lose lots of power, requiring more thottle to maintain speed. Very annoying. Disconnecting the ARV eliminated the problem, and it ran like a scalded cat from then on.

- JimY