The Fuel Pressure Relief/Overflow Valve has some small plastic piece inside that warps or breaks. If that happens it won't hold the Fuel Pressure as well during idle.

When the Injector Nozzles were replaced where the pop pressures on the injectors set and balanced?

Does it need Motor and Transmission Mounts?

Uneven compression can cause the uneven idle.

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84 300D, 82 Volvo 244Gl Diesel