Is this car new to you? If so, I'd check to see that the air filter housing mounts were good (mine were broken!). They are little rubber gizmos that have a bolt on both ends -- one goes to the nut holding the bottom half of the housing in place and the other into the head or intake. If they are broken, you can get dirt in the intake -- read worn out motor!
All the rubber parts in the engine compartment will die on these cars with heat and age -- I've replaced all the rubber connectors on the various vacuum and manifold pressure signal lines, and the idle valve hoses. Makes a huge difference!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!