Air in the fuel inlet line. If the governor, it could be the O-rings on the shaft causing a vacuum leak.
I don't know if the old method used on gassers would work to find a vacuum leak. You spray something like starting fluid around the suspected leak area and if the rpm changes you know it got sucked in through a leak.

If the vacuum pump diaphragm goes bad oil vapor gets sucked into the intake manifold. I don't remember the symptoms.

You could do the injector cut out test and see if one injector or cylinder is having an issue.

The fuel pressure relief/overflow valve springs becomes compressed over time and effects the low-speed fuel supply pressure. If you have one that can be taken apart the spring can be stretched to a 27mm free length and reused.

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84 300D, 82 Volvo 244Gl Diesel