Phreakish said

AFR, or Lambda alone won't tell the whole tale. Here's a good read that illustrates some concepts.
It's entirely possible the AFR is lying and that something else is going on. I've seen plugs with internal defects, impossible-to-see cracks in porcelain, or which just plain don't work under load cause a random misfire. I chased an issue like this on an engine once which would buck under high load in high gear, but nowhere else and once above 3k would be fine (and I'm sure the opposite is possible too).
Point being, like PRH is alluding too: don't lose the forest for the trees. If you can get some additional data from a gas analyzer, it might help, but I'd be swapping plugs and wires around to see if the issue follows them, or if new plugs/wires (or routing) addresses the issue. I'd even be looking at the pickup to ensure there isn't a mechanical cause for a misfire.

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