I second what jollygreen said. It is probably the caps and rotors. Classic symptoms. What I have found is that moisture will make its way up into the caps and corrode them if the car is not driven regularly, especially if you are in a humid climate. Driving the car a lot may clear up the problem, but it's best just to replace them. Cost is pretty high, but if you shop around, you can get the Bosch part (exactly the same as the MB part but without the 3-point star) for a lot less. The good news is that they are very easy to replace. It takes me a very leisurely 2 hours to do both sets. Might as well replace the plugs while you are in there too, since the plugs should be done every 30K miles also.
The caps and rotors are about the only semi-weak spot on these cars. Most frequent cause of driveability problems I've found. I say semi because all that's really needed is to replace the caps and rotors every 30K miles and you are good to go. Slightly higher maintenance and expense than the later coil pack E420's, but not bad at all. These are still awesome motors. Driving the car regularly helps too.
1972 280SE 4.5
1984 300SD, 250K
1991 300SE, 89K
1992 300D (330K miles when KIA'd by mother-in-law 8/10/09 - still sitting in my barn)
1994 E320, 109K
1995 E420, 60K
Proud Member of the Mercedes Benz Club of America - Idaho Chapter