Have someone press the fuel pedal to the floor.
check the injection pump to see if the linkage has advanced the arm on the pump fully. If you can still move the arm further manually it is not getting to it's stop by the linkage. Repair or adjust the linkage. The pump arm has to reach its stop with the linkage.

I also forgot to mention that you should change the fuel filters if you have not. They may be partially obstructed. This could be basically starving the engine for enough fuel to get up and go. Check the linkage first though.

A much better test would be to read the base pressure in the injection pump. A shortcut to that is to remove the return line from the injection pump and see if fuel is coming out with engine running at idle and reving the engine a little.

This is not as conclusive as establishing you have good base injection pump fuel pressure by actual measurement. Yet if there is no flow from the return you know you are starving for adaquate fuel.

Another test for inadaquate fuel is most of these cars will emit some black smoke if pushed hard. You could have someone follow you and tell you if any is present when you accelerate hard. I suspect in your case there is none at present.