V6 sudden misfire

Hi all well not had my freelander a year yet and another problem has cropped up. It has been my daily drive. I had problems with the cooling system to begin with but that’s sorted now. Yesterday I drove approximately 2 miles and a misfire developed ( I don’t hang around when driving ) when I got home did a code scan got the cod p0303 powertrain cylinder 3 misfire. Removed hot lead and found the spark plug well was full of oil and the rubber on the hot lead was falling apart so went and ordered a rocker cover gasket to stop the oil leak and a new set of ht leads. I removed the spark plug and it looks very dark on the eclectrode I would say dry fouling ??? Strange as she has ran so smooth. I put the new hot lead on today and started her up seemed ok but still a little lumpy. Went for a drive and still a misfire not as bad but still there. Engine management lite flashing every so often ???? Any ideas how this suddenly happens. No white smoke from the exhaust or blue smoke however today I did notice a smell of burning oil under the bonnet and a bit of smoke ????? I’m hoping this is not a serious problem I’m hoping someone can shed some light on this for me thanks in advance. Also seems to pull well when I really put my foot down and accelerate hard