Check the Glow Plugs and you could have a small Air leak.

The Air leak would be in the Fuel Supply system.
The Injectors are in the Highe Pressure part of the system and there would be high pressure Fuel sprayed from an external leak.
In order for the Injectors to lose their prime so to speak the Nozzle seats would have to be badly worn or the Delivery Valves in the Fuel Injection Pump would have to be letting the Fuel seep past them.

If you are running the Stock Bosch Fuel Injection Nozzles there is tiny drilled passages in the Pintel of the Spray Nozzle. And, they can get plugged with Carbon and cause a rough idle.
Look in one of the rough idle threads where I have pic/s.

MW Fuel Injection Pumps do not hav the Delivery Valve Holder O-rings. But, some had similar symptoms when their Delivery Valve O-rings started to go bad. The claim is that somehow Air is pullled past them.
There are some threads where changing the O-rings solved the Problems.

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84 300D, 82 Volvo 244Gl Diesel