Here is my guess: I don't think it is plug rleated UNLESS a plug wasn't check for a proper gap.
It could instead be related to a wire or connector.
I hate to admit this ...... OK here goes ..... I was going crazy trying to figure out why the 300E idled roughly every so often. When I recently changed the plugs, the loose spark plug connection on cylinder # was the obvious reason.
Check each connector. Twist them gently and tug on 'em to make sure they are on securely.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)