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A crack in an ignition coil's insulator letting the spark track to earth instead of jumping the plug gap. The spark takes the easiest path, and a carbonised track across a cracked body is easier than a plug gap under compression.

So the misfire appears under load and not at idle, because cylinder pressure raises the voltage needed at the plug while the track's demand stays constant. A thin dark line down the insulator is the visible evidence, and tracking is often audible or visible as a blue flicker in the dark with the engine running. Oil in the plug well accelerates it by attacking the boot, so check the well before fitting a replacement — a new coil into an oily well fails the same way.