This may not be your answer,but it certainly something to watch for.One thing I found while checking spark plug color was that the coil packs are not always sealed to the valve cover very well,water proof. It causes all kinds of weird surging issues, and can eventually cause misfiring. You say " Oh well the coil failed, and the plug is fouled." and that would be true, sort of. The coil hasn't failed, but if you remove the boot from the coil, most of the time they corrode pretty badly inside that boot, and where it contacts with the coil.( it looks like a spring) This is especially true if you wash your engine often, or have a hood scoop. Dip the boot assembled, ( don't remove the spring ) in a zinc phosphate bath over night. Also with the bootless coil laying on a work bench upright, take an eye dropper and put a few drops of zinc in there too. Zinc is a good conductor so I think it will be safe, and the phosphate should remove the corrosion pretty well.