#6 cylinder fouling plug

hey guys new here anyway ive looked thru 50 pages on this fourum and dont see really what im looking for...... i have a 1996 z-71 350 motor it has about 126k on it and the motor is stock (k&n air filter) the #6 plug keeps fouling i have new plugs,, wires,,cap and rotor,, manifold gasket was changed about two years ago due to leakage... this has been going on for about two months now i really need to fix this the performance is just not the same as it was ,,, no codes are showing at this time