One of my neighbours is the owner of our "old" 1995 C36. (One of our other neighbours has our "old" 1997 BMW 528i. A trend?)
Last night while doing some work outside (shoveling SNOW) I aksed him about the car. He loves the C36, but had the same problem you're describing a few months back. The dealership replaced the coil ends ($140CDN p/l) and it was the cure.
So, I put in another vote for the coil ends.
1998 C230 "Black Betty"