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This may be a simple plug wire and ignition coil issue. Sometimes a spray bottle with water can help determine if it's the problem. With the engine running, spray some water on the ignition coil. If the misfire happens or you can see sparks arcing then you need to replace the coil.
If the spark plug wire had oil on it where it connects to the spark plug, this could also be the problem. The oil makes the rubber soft and then the spark can arc through it to the cylinder head.
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