Without giving away any details, I have a friend who has a 1982 300D. It has no problem pegging the US spec 85-mph speedometer. Well in excess of 300K miles.
For the most part your problem can be broken down into several classes:
1) Not enough fuel delivery. Could be filters, could be injection pump and its ALDA control.
2) Not enough air delivery. Clogged air filter, bad turbo.
3) Insufficient compression - excessive blowby. Can be tested with compression gauge. Would probably show up as hard starting.
A 240D should do 80 mph, even 85.
Please fix this before you are passed by a 240D. It's a matter of honor.
Ken300D