What vacuum have you got at the boost solenoid? (Should be around 25" if I remember correctly).
It's at the rear of the engine underneath the car (just around/above the offside driveshaft bearing).
Check vacuum in, and good vacuum at the turbo. The pipes are prone to going soft with oil leaks, and bleed off the vacuum.
Use a mityvac (or suitable vacuum pump) to check the lever on the turbo moves smoothly, and to the full extent of it's travel. Not unusual for shopping trolleys to never go the full extent, eventually carboning up.

And of course, check the boost pipes and intercooler aren't leaking. Oh, and the pipes are on the boost solenoid correctly. One is vac in, one is vac out, the other is vacuum to atmosphere
with key off.