The plug is probably fouling because that cylinder is burning too much oil. This can either be caused by oil leaking through from the valves or worn rings. What is the oil consumption on this car? It is possible that there is only a problem with that specific cylinder if that is the only plug that is fouling and would account for why the overall oil consumption is not too high.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record