Well, FWIW, since it isn't the EGR valve (since he's bb'd the line from the SOV), and since it isn't the wiring harness, I'd be inclined to check your spark plug / coil wires again, and probably check the connection to the wire ends.
After that, I'd suspect the mass air sensor or the throttle actuator. Both are not uncommon failure items on these engines and both will affect idle. A vacuum leak will also cause these two items to act up.
Just hope it's not the actuator. It's about $961 if you order it from this website and a lot more at the dealer. I think the mass air sensor is a little over $200.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".