Mine does the same thing now and then (maybe once in every 300 miles). We've attributed it to the wet conditions, never does it during the dry parts of the summer

Squirt of WD40 into the plug cap and a clean out, give the contacts a squeeze and refit. There is usually some crap comes out of the plug cap, but I couldn't say if it was the cause of the problem.

Perhaps, at such high engine revs, coupled with a less than perfect plug-cap-fit you are geting a split second where the spark isn't happening ? Or maybe not!

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