I had a problem a while back with my E320 stmbling very badly during warmup. It turned that the problem was quite general here in Western Washington, and my Mercedes dealer had considerable experience with it.

They recommended using a high concentration of Techron additive. Use your gas down to less than a quarter full, then add a full, large bottle of techron additive, then go out and drive hard. Try to go up long hills at full throttle, etc.

This process worked very well for me. It cured the stumbling completely. Now I just fill up with Chevron occasionally, and the problem has not returned.