I had the same problem. I had a car shipped in from out of state. Drove it home from the drop off point and it was a super dog. 85 300D I thought something was seriously wrong, especially after I drove someone elses 300D which was night and day different from a power perspective. I finally found the problem. The vacuum hose mentioned in the first response had popped off the banjo fitting at the rear of the intake manifold. I pushed it back on and the car took off like a bandit. Look for a danging hose in the vicinity of the rear of the valve cover. Mine kept falling off so I replaced it with some EGR plastic vacuum line I found at Advanced Auto and I have never had an issue since.

Don't give up on this car. Although they built like tanks, they do have small issues that will develop from age. This site really helped me flush them out and address all of them. If you're handy, you can find parts relatively easy and the repair costs are reasonable.