I would not rule out a possible the Air Leak issue.
You could put a longer Hose in place of the Cigar Hose and submerge the other end of the Hose in Diesel Fuel to see if Air Bubbles are coming out.

Fuel Supply issue. Like the Fuel Pressure Relief/Overflow Valve and a worn Fuel Supply/Lift Pump.
If you have a Parts Car you could swap the Over Flow Valve and eve the Fuel Supply Pump. Do one at a time so if one fixes the issue you know which one caused the problem.

If the EGR is still there an functioning is it possible the EGR is coming on when it is not supposed to?

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84 300D, 82 Volvo 244Gl Diesel