Hi, I'm not an expert with NSR's but I would suggest that it is electrical.
Even though you are getting a spark at the plugs, it may be intermittent. It sounds to me like a contact breaking down somewhere maybe an intermittent short.
Carefully check and clean all the contacts in the ignition circuit....especially the connections to the ignition coils. If it then persists then swap the ignition coils (and plugs) and see if the problem changes to the other cylinder.
If it does then bingo. Ignition coils are not that expensive.

Of course, I may be well wide of the mark...but it's worth checking anyway.
Good luck.
Mike
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