98 mustang idle problem


my check engine light went on one day on the way home from work and i instantly lost some power. its a 4.6L SOHC engine. i have worked on diesle wiel i was in the military and always tried to learn as much as i can about cars. so i do most of my own work and do it well. so i dived in. started with a visit to auto zone for a quick diognastic. it cam back as a no or miss fire. i changed spark plug wire first and had to wait for auto zone to get in some plugs for my car. while i was waighting i took advantage of the time and put in MSD coil packs, new injectors, cold air fender well intake, pcv valve. the basics. i also have a 75mm throttle body and high flow plenum and bassani complete exhaust. so after all that i had the plugs and i figure that that will be my last fix and the check engine light will go off. well i get to my last plug on the passanger side witch is number 1 on the firing order. when i took it out i noticed oil on my socket and on the plug itself. this is not good. now im not sure if its do to the piston charging and not firing or if i have a bigger problem. my check engine light is still on and still lost power. do i need a new head gasket? or is it worse than that. i was looking to go up to PI heads. is now the time?