When we bought our 83 300SD it was very sluggish. I found the lever on top of the valve cover that connects to the cable on the transmission to be rotated 180 degrees and not pulling the cable. I believe this cable downshifts the transmission from 2nd to 1st gear when you depress the accelerator. I disconnected the cable, rotated the lever and hooked the cable back up - fixed the problem. The previous owner had complained to the dealer who had installed a rebuilt transmission when they purchased the car. The dealer told them diesels are just slow like that. The PO had been taking off in 2nd gear for three years!!!