Clean the line from the intake manifold to the ALDA on top of the injection pump.

Temporarily bypass the overboost switch. Sometimes the boost sensor sticks.

Check that the air recirculation thing under the EGR valve isn't stuck open. A quick check is to plug the line to the recirculation valve. That doesn't help if the valve is physically stuck open, though.

Sixto
91 300SE
87 300SDL