Perhaps your timing chain has jumped a tooth and is causing the rough idle. That was the problem on my 380SE. I had dealt with a rough idle for a long time, replaced injectors, fuel head, lambda tower, plugs, wires, cap, rotor, etc...and still couldn't get it right. Finally I decided to pull the valve covers and have a look inside...
Well what I found brought somewhat of a sigh of relief to me. I saw that one of my guide rails had broken at the top and that the chain had at some point jumped on the cam sprocket, but not enough to damage the engine (the sigh came here
This was last year at 110,000 miles so I decided to do all upper rails, a new chain and a new tensioner (original MB ONLY!--I've seen many problems with the Febi Bilstein tensioners). Spent the day rediong everything, and at that time switched over to Mobil 1 synthetic, jumed in the car the very next day and took off for Atlanta. Ahh, it idled so smoothly and since the switch to synthetic, it returned about 25+mpg even while travelling 85-90mph will four people and a trunk full of luggage! So, since 420SELs are known for timing chain problems, I'd pull those valve covers and check everything in there. Might save you some money and headaches.