Well I am going off of other posts on this site, but everything I have read on here about the Bosch +4 plugs is that they suck on MBs. You may do a search (small blue bars above second from right) on this topic and see what you find. If I was you, I would replace the plugs with OE plugs. I'm not saying that this is the cure, but it is a good start. Also, the rough idle is a common problem, do a search on that too and see what you find (you'll find a lot!). Good luck.

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'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."