Mercedes dealers have been "superceding" spark plugs on some cars to, as far as I can tell, manage their inventory a bit better (maybe fewer plugs for more cars, I don't know). The problem is that when you go to the dealership for, say for example a W9DC, you may walk-out with a W9DC0. The 0 in this case means it is a different plug (larger center electrode) and for example on my car, it caused a hellish idle problem. The engine miss was so bad that the hood bounced up-and-down as you described. After putting back-in W9DC's the problem went away instantly.

The moral to the story is to take a magnifying glass (figuratively, not literally) to the exact part number on your spark plug. If it is not precisely the original Bosch part number that is in the owners manual and originally sold with the car, then change it.

After checking-out the other fundamental issues in the ignition system (per other posts)-- distributor, rotor, wires, etc, you might try some fuel injector cleaner and running the engine for a nice hard ride ASSUMING that your acceleration and driving is miss-free, ping-free, and knock-free (the engine is running well when normal driving). I like Chevron's Techron fuel injector cleaner-- I recently found a vacuum leak and corrected that (another possible cause of your problem but you should exhaust all ignition possibilities FIRST) and also put some Techron in there and drove the car hard. The combination of the two (or just one or the other, I'll never be sure) increased gas mileage incredibly and allowed me to lean the car back-out at idle to maintain a smooth idle.

The order of troubleshooting for a problem like this I think best is 1) Ignition 2) Fuel System Cleaning (e.g. fuel injector cleaning) 3) Fuel system operation and Vacuum. Some mechanics like to sell you new fuel distributors, injectors, and manifold gaskets so be careful in step 3.