It's electrical, could be the coil, if you have a look down the page there's a guy with a stuttering TBS with similar symptoms which look like he's lost a cell of his battery.
If your battery is OK then check all around the electrics and connections but sounds like the start of coil trouble when they have warmed up. If you're lucky it could have been condensation which has now dried out and it won't appear again !
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Ride on !
