If you ever went to an interview for a Desel Mechanics Job one of the first questions they are going to as is if there is a lose of power what is the most likely cause. The answer is the Fuel Filters are plugged or there is some other issuel with the fuel supply system.
In your case that includes the Filters, Fuel Tank Screen, the Fuel Supply/Lift Pump and the Fue Pressure Relief Overflow Valve.
Additionally the Fuel Tank Vent could be restricted our plugged.

For the Fuel Tank Vent have about 1/2 tank of Fuel and see if driving without the Fuel Tank Fill Cap on. If it runs better it is you tank vent clogged.

Again you need at least 1/2 tank of fuel. Swap the position of the Fuel Supply Hose with the Fuel Inlet Hose. If that fixes the Issue your Fuel Tank Screen is plugged. (Don't drive it like this as you will run out of fuel before it shows that on the gauge.)

If your Fuel Pressure Relief/Overflow Valve is they type you can take apart you can take it apart and stretch the spring inside till it has a free length of 27mm.

You can rebuild the Fuel Supply Lift Pump for about $32 for the Kit.

When I first got my Car the throttle Linkages were messed up and out of adjustment and my Car was deadly slow. But in miy case once it got going it ran normal.

Besides the linkage there is that part on the fire wall that the pedal and throttle linkages pivot off of. The Rubber and plastic parts on that fall apart.

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84 300D, 82 Volvo 244Gl Diesel