Are any of the other plugs fouling too? Is it possible you are running too rich in that cylinder? I had a rough Idle in my 420SEL.Turned out to be my secondary air pump. This is located right below your alternator on the passenger side front of the engine. It just bolts to the block, and si driven by a pulley. There are two hoses. One draws air from the air intake assembly on the top (hose goes in on front passenger side of the box). The other hose pumps the air into the exhaust. The connector holding that connected the hose to the engine block had broken off, and I didn't realize it. It went for several months whithout my noticing and all along I had a rough idle, particularly when cold. The car would shake like it had a really bad miss. They replaced the plugs, ignition coil and scoped out everything. It didn't fix it. Because this hose was off the air pump got water in it and rusted out. I got a used on installed, along with a new hose and connector. Used pump cost me $350 Cdn as opposed to $1200 new from the dealer. Installation and the hose and valves were about $300. Now the car no longer idles rough anytime! I was going out of my mind wondering what was causing the rough idle. I couldn't figure out how with so many tests they couldn't find the problem. I finally asked my dad what could cause a rough idle. He said it sounded like the engine was running rich if all the other things had been checked out. I knew the mixture was fine because the had just adjusted it so I could pass emmissions. I guess the air pump, which only opperates for a few minutes pumps extra air which leans out the mixture until your engine is hot enough. I'm not a mechanic so I'm probably not explaining this correctly - feel free to correct me anyone - but I do know that now the car idles smooth as silk. Check your air pump and the hoses/conections/valves from it. A visual inspection should do. 2 minutes to check it out could fix your headache. This doesn't explian the fouled #8 plug, but it might be worth a look.