Yeah, the coils can break down at high RPM, so once you've checked the cap, (or replaced it to be sure, they're cheap enough) then try swapping the coils to see if the fault swaps to the upper cylinder.

Another thing whilst you're there, is to check the connectors for the coils. It doesn't take much for the to get a bit loose and start to go black and pitted. This builds up resistance and causes bad running. Give 'em a good clean.
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