I'm glad you found the problem. This happened to me a month ago. Changed my oil then the rough idle. Changed plugs, no improvement. Thought it was the wires so I tried to rebuild the tips to not avail and ended buying a new set. The rough idle persisted... Performed Italian tune-up and viola!!!! It appears the #6 plug well or plug had gotten wet with oil. I'm still stumped on why it did that. At least I know now that the oil burned off and all is well.
FYI... the wire tips have a 1Kohm resistance while the plugs (H9DC) don't or have a small amount. The platinum plugs have their own resistance and that's why it's not recommended as per many of the posts. As for Japanese cars, the resistance is in the plugs and not on the wire and that's why their wires are so readily avail I suppose.
Oh yeah, use dielectric grease on the plugs so you can remove the boot easily. Never pull the wire by the wire. I think there's a special plier for removing the boot from the plug... but it's like $50.
'87 W124 260E (DD)
98K orig. mi. @7/15
CLK 7-Spoke Forged Wheels
Neuspeed springs/Bilstein Sport
4/3 bump (F/R)
'97 993 Carrera
106K orig. mi.
Always driven like it's stolen