Could be a few things. Might want to run Molly Lube injector cleaner through it.

Could be your kickdown switch, as the gas pedal. Mine was sort of stuck, prevening the car from shifting and accelerating properly. They also hit the fuel linkage with some lubing kind of stuff so it moved easier. The car was much quicker after this. Mine's a 1991 300d.

Make sure your air and fuel filters are good, so the car gets enough air and fuel to work properly.