Replacing a spark plug again?

My car engine started rattling so I replaced all of my 6 spark plug(1 of the spark plug was black and greasy) yesterday and my car run pretty smooth but afterwards the engine just shut off itself when I stop at a red light for a while, so I unplug and checked all the 6 spark plugs again and one of the plug is completely black and greasy again, what should I do? Replace it again? What if it becomes black and greasy again? It's of no use!?! :shakehead

Sounds like that cylinder may be damaged. I recommend performing a compression test, actually several. One dry, and one wet to determine ring condition. One at TDC and one at BDC to determine bore taper. Damaged rings or excessive taper can allow oil to blow past the piston from the crackcase into the combustion chamber.

Also check the plug wires. The #6 wire may be failing. However, even if it was firing properly, it sounds as if you engine would be burning oil anyways, because of a defect as described in the above post.