I would recommend that you should buy a new Ignition Wire set. They wear out and this set is cheap to replace. I use these guys for parts and found that they are very efficient and ultra-reliable:

http://www.***************/search/product.aspx?sid=bvy5l0i5nqd5mr45emwrwp55&makeid=800016@Mercedes&modelid=1195422@C220&year=1994&cid=25@Ignition%20Parts&gid=7228@Spark%20Plug%20Wire%20Set