Wow, first off you got one heck of a spot to do a plug chops
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I heard the slip ??? that's what it sounded like anyways and that's what I would think. But you say the clutch all checks out. Fibers are good, plates are good ? Have you tried adjusting the clutch at all ? It really sounds like the clutch is slipping. Have you looked back and made sure the tires aren't spinning ?
I know you go through things well but I have to ask, is the float set correctly? main jet plug chops coming out well ? Running through the fuel quicker than the bowl fills you will get a "lean burst of power" for lack of better words. But it would usually bog out if ran continuous high rpm.
Yours just hits that one spot and then clears right back up?