+2 on B2 piston possibility.

Many options for injector bypass hose. 1/8" fuel hose works.

It also helps to know what you are working on. W123, W124, W210 ?

Funny, my first thought was that any 300D would run funny in a lake !

Note that there is much info about tuning and diagnosis of engine performance here.
What is your mileage? Is the fuel clean?
How old are the fuel filters?
How old are the hoses? Any vacuum leaks?
Maybe the injectors need to be rebuilt/replaced?
You might need to rebuild the delivery valves?
The vacuum line from the banjo bolt on the intake around to the alda or alda control on the firewall (forget exact name if this part), may need to be cleaned.

Also keep in mind that a bad motor mount will give the illusion of issues. Make sure the mounts are healthy.

__________________
85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.