Check engine light came on, read code, cyl #8 misfire, replaced all Plugs and Coil Packs, ran great for appx 2 weeks, started running badly, read code, #8 misfire: swapped #4 with #8 etc, reset codes, read code again, #8 misfire. Any suggestions or thoughts? I do not understand "why" it ran so good for appx 2 weeks and then #8 misfire code again. Even after I swapped #4 & #8, getting #8 misfire? I'm puzzled. It's running very badly. The spark plugs were not fouled or oil covered either time. I do not know what to do next!

 

Make sure the plug on the coil is tight and making good contact. Plug connections that were loose were known to give this condition. Also check for any possible vacuum leak at that location, all hoses.