Does your car have a Pneumatic or Mechanical Governor?
The Mercedes with pneumatic Governors have a Butterfly Valve in the Intake Manifold and a hose/tubing going from the Governor to the Intake Manifold.
Inside of the Pneumatic Governor is a Diaphragm made of oiled Goatskin (that is what I have been told). These Diaphragms get stiff as the bet older (they are also almost $200 each just for the part). There is some talk about them in the Vintage Mercedes section.
Also the Vacuum hose to the Governor and the port that is open to air in the Governor can become plugged or restricted over time.

You did not say how many miles are on your car or how many miles it has been since you had the Fuel Injectors Checked.

The Timing Chain stretch that the other member was talking about causes late/retarded Camshaft and Fuel Injection Pump timing.

Evidently your Mechanic does not know how to check the timing on your Fuel Injection Pump.

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84 300D, 82 Volvo 244Gl Diesel