Here's my problem:
I have a '03 w211, E500, engine light came on and it was running rough.
I put gummout fuel injection cleaner in and eventually it ran smoother and eventually the light went out.
after the gummout/fuel was out and I refueled, it started running rough again and it started the same thing again-so I put gummout back in, and it eventually smoothed out.
When that was out of the tank, it started running rough again.
Am I missing something?

Is the CEL on? Did you pull any codes?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Yes it is on. It goes out when I put gummout in and the code from it says misfire in cylinder 8. I changed the plugs, coil, and wires before I put the gummout in.

I am guessing it has 4 coils, is that correct?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Sorry it took so long for me to respond, but no it actually has 8 coils, 16 spark plugs.

I wonder if it is possible the code is wrong in some respect. Either calling out the wrong cylinder, or reporting a symptom of another problem. You could try swapping the coils around and see if the code changes. Have you searched? You might find others who have gotten this code and what the trouble turned out to be.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Check your plugs, the injector cleaner could have fouled them. 16 plugs is going to be fun to check.

(also, mercedes specifies to not use any form of fuel additive for any reason other than weather conditions)