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I have a nice photo of this but I can't figure out how to attach it so I'll have to go with a description of the problem.

I'm not a stranger to reading spark plugs but I've never before seen ones like what I just pulled from my Tohatsu 8B. The plugs are squeeky clean except for carbon deposits on the anode, the very tip of cathode, and a strange stripe of deposit on either side of the insulator that looks like a fat lightning bolt. Both plugs are the same.

Anyone got a theory about this? The motor runs and makes decent power but it stumbles a bit and doesn't hold a steady rpm.

Could be a lot of things but sounds like a misfire. Have you tried replacing them?

I tried cleaning them with varsol but it appears the pattern on the insulators is burned right in, not just a deposit. So I think you're right. I think they're cracked and arcing through the insulator. So, yes, I will replace them.

It seems very odd to me that they both cracked essentially the same way at essentially the same time. (They were installed new less than 10 hours ago.) But I can't think of another explanation.

did you replace with the suggested brand and gap them properly? I had misfires like this on a car but it was after 60k miles so that was the result of use but it was a streak along the side and I was having power issues after 3rd gear. Also a lot of things could contribute like fuel to oil mixture, improper idle speed (idling too low for long periods) or air to fuel mixture. I would replace them and double check the gap and hopefully that is it. Spark plugs are cheaper than mechanic rates.