Check the boost lines and the banjo bolt coming off the back of the the manifold. They can all get plugged and cause a lack of added fuel. My 300SD was sluggish when I first got it and after a combination of running it a few hundred miles on the highway and cleaning the black sludge out of the banjo bolt and portion of the line it got much more responsive.
It may also be time to check the chain for stretch. A worn chain will give you late injection timing and valve timing. Late valve timing can also make pickup bad at low speeds.