Too much crankcase pressure will also shut you off. Too much crankcase pressure will push the Vacuum Shutoff Diaphragm towards shutoff.

Try cleaning out your Crank Case Breather tubing. Or, if you do not mind the mess try a drive with out the Tubing on and see if the problem goes away.

It also could be related to the Fuel Supply/Lift Pump and Pressure Relief/Overflow Valve not keeping the fuel supply pressure high enough at lower rpms.
Along with the above is your idle speed 800-850 rpms?

Valve Adjustment?

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84 300D, 82 Volvo 244Gl Diesel