My '85 300D had a similar problem when I bought her three months ago. Like others have said, I would check:

1. Both fuel filters,
2. The fuel lines leading to the filters to make sure they are not twisted or kinked,
3. Both banjo bolts (at ALDA and behind the intake manifold).

On my car, there was a kink in the line leading to the pre-filter, and my banjo bolts were completely blocked. All the above are easy and cheap fixes, and very effective in improving power and performance. I can now reach 60 MPH with no problem, and there's plenty of power to accelerate further. A diesel purge will make a huge difference too.