I'm not sure if you read my post of my tormenting E320 but I just resolved a stumbling issue, that's been driving me crazy all summer, by changing the spark plugs. I too was running the Bosch Supers and they had about 16k miles on them when the problem started.
I put a cheap set of Champions in it and it's been smooth as silk ever since. I know the Champions have resistors built into them but I'm not sure I'm going to trust another set of the Bosch's after my recent hair pulling, head scratching experience.
Makes me wonder if Bosch put out a bad batch of Supers or if they just don't normally last very long. HMMMMMM