So far it sounds like an Air Leak. When you changed the Plastic Primary Filter did you also change the Fuel Hoses there.
I found on mine that the old Hoses were hard enough that even after I tightened the clamps I could twist the Plastic Filter by hand.
Other thoughts
If before you did the Motor Mount change your Fuel Hoses going to the Fuel Supply Pump were sort of tight; raising the Engine up could have pulled them tighter and maybe done something to the plastic tube that the rubber hoses attach to.

Another source of an air leak can be the Hand Primer. If you have the old style Aluminum tube one with the hard plastic Knob besure to push it all the way to the bottom and turn the Knob clockwise and tighten it in the down position. This is supposed to seal it at the bottom.

It is normal to have a bubble in the Plastic Filter.

You did not say if the miss goes away after you run the car at higher speeds or not?

Run the car with out the Fuel Tank Cap on to see if it is a tank venting problem.

Swap the Fuel Inlet hose with the Fuel Outlet hose. This will bypass the Fuel Tank Screen. If your car works better when you do this the Tank screen may be plugged.(I think you need at least 1/2 tank of fuel for this to work.)

When was the last time the Valves were adjusted?

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84 300D, 82 Volvo 244Gl Diesel