No one knows what's wrong w/my C320

[kris-benz]

I have a 2002 C320 that has driven great up until now. The check engine light came on two weeks ago and I took it to an independent mechanic who specializes in Mercedes. The car was idling and vibrating at stop lights. The codes said Banks 1 and 2 were too rich. He said it needed a new mass air flow sensor and so I replaced that. About 20 minutes after I drove it off the check engine light came back on so I brought it back in and they said the code came up for some type of emissions control and they spent over two hours checking it over and all they found was a small vacuum leak which he fixed. A couple days after that the check engine light came back on and the car was again running rough. It was idling and hesitating at stop lights and also making a hissing type noise when you accelerate. I brought it back in and they spent eight hours looking at it and could not find what was wrong. The same codes came on as before--being too rich. He thought it might be the spark plug wires even though the car only has 47,000 miles on it--he said a couple looked like they were old. He suggested replacing those and/or the spark plugs and see if that helps. He checked everything--the catalytic converter, the exhaust pipes, and can find nothing wrong. He wants me to bring it in Monday to see about the spark plug/wires. I am so frustrated with this car. Does anybody have any suggestions? Should I replace the spark plugs and/or wires? Would the dealer know more?