In conventional applications, the turbo pressurizes the intake manifold and the fuel system provides fuel to match. The turbo is lmited in how much pressure it can produce. MB turned the tables in this car - the ECU decides how much pressure the turbo puts out. You can get into what inputs influence the ECU, how the ECU rationalizes these inputs and how the ECU effects its decisions. Many here have effectively cut the ECU out of the loop by retrofitting a conventional boost operated wastegate in place of the ECU controlled vacuum operated wastegate. This modification will cost you about two hours of dealer diagnostics.

My best guess from the information you provided is the temp(?) sensor that informs the ECU that the EGR system is operating is clogged with soot from 300K miles of proper EGR function. When the ECU thinks the EGR system is not functioning as it should, it penalizes the driver with reduced or no boost.

Take a look inside the engine at what EGR has done over the years and determine whether the environment is better servied by another engine or entire car in a wrecking yard and the resources expended to afford you a replacement, or a properly functioning car with EGR disabled.

Sixto
87 300D