Your case is so similar to mine. I also have an S320 1994 and recently I decided that I was going to change the spark plugs myself. I did the job and everything was fine. A few days after I noticed that the car was shaking a bit and decided to check if there was something loose or so. I noticed that one of the boots was cracked. After disconnecting and connecting all the wires again the car was fine. I decided to buy the new boots and replace them. After replacing the boots I started the engine and the car was shaking very badly. After connecting and disconecting the coils and the wires several times times the problem got worse. It came to the point that the car did not start at all. I took it to the mechanic and found out that it was the engine wire harnes. I think that you should not use the car untill you check out this harnes. At my mechanic there was C class mercedes that had a bad harnes and it damaged the ignition system, instrument cluster, computer, etc. I saw the old part after it was replaced in my car and I was impressed of the poor quality and the incredible bad shape of the wire insulation. As a matter of fact I preparing a letter and I am also sending the part to the Mercedes Benz Engineering Department. Good luck and check out this part, it seems to be a common problem.