I went through months of hell with the same problem. I changed everything without success. And I mean everything!
The final fix was one new coil. There is no test on the coil I can do to prove it was bad...it is just that the car now runs a sweet as a new one.
You have three coils. I suspect that one is bad. You have to try each of the three positions or buy three new coils. They cost $35-65 each on eBay or online Merc stores.
http://www.***************/search/product.aspx?sid=jw52q2nbux2s0tvwq024kd55&makeid=800016@Mercedes&modelid=1195499@C280&year=1997&cid=25@Ignition%20Parts&gid=7060@Ignition%20Coil