Several days ago, I solved my 400E's cold rough idle problem by simply using some Pennzoil fuel system cleaner, the idle smoothed out the day after I added the cleaner, before I had invested $1000+ for the parts and many weekends for this annoying rough idle.
In my case, it seemed carbon deposit was the cause. My wild guess is that because of their extra high gear ratio (1:2.24), I seldom need to rev the engine to above 3000 rpm unless I want to drive aggressively, this might cause carbon build-up on intake valves and within combustion chamber.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.