It seems you have it narrowed down. As a next step I might suggest a new spark plug wire on that cylinder, drive it and see what happens. My other thought: if the cylinder is fouling the spark plug in a short time frame as you described then you are experiencing an obvious mis-fire. You might want to have a compression and leak down test performed on that cylinder. This will help determine if the cylinder is mechanically capable of igniting the air/fuel mixture for maximum efficiency. Low compression and or high leak down will be a good indication that "you're going in!"