After it has sat a long time try pumping a lot on the Hand Primer. If it starts OK after that you are having a Fuel Supply issue where the Fuel is migrating back to the Fuel Tank; sort of the Pump losing prime.
That could be an Air Leak or it could be the Fuel Supply/Lift Pump Valves are worn. If it is the Valves Fastlane sells a kit that does not cost too much.

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84 300D, 82 Volvo 244Gl Diesel