Maybe you can try Pennzoil fuel system cleaner, it cured the rough idle problem on my 400E.
I had experienced the same problem on my 92 400E, I also couldn't find BG 44K that Michael recommended, I then used Pennzoil fuel sytem cleaner instead, the rough idle problem was gone right after an Italian tuneup.
For details, you can search the thread post by me a couple of months ago.
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.