I'd be willing to bet you have a fuel enrichment problem at boost. Your turbo may be boosting, but the pressure signal from the manifold that goes to an electric switchover valve mounted to the firewall behind the brake booster then to the ALDA on the injection pump may not be making it through. So in effect your ALDA is receiving zero boost signal thereby not adding more fuel as boost increases as it was intended to do. You could have a plugged up banjo bolt, plugged pressure tubing, or plugged/inoperative firewall switchover valve. Also, the boost pressure sensor on the intake manifold could be faulty. Replacing these is easy and fixed my problem which was almost exactly like yours.
Can you feel the effect of the turbocharger at around 2000 rpm?

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