You know that every time I look at a plug I'm going to be seeing that now. :doh:http://farm7.static.flickr.com/6140/5918773221_4b8b6650c9_z.jpg
That image concerns me. It looks like there are a small speckle like deposits on the plug. If this is the case that can be a sure sign of severe det. Essentially your piston crowns melting and depositing on the plugs.
I may be wrong but just to bring this to your attention - just in case.
Does it look a bit like welding sputter?