I have a 2000 xj 4.0 and I am having a problem with cylinder #5. I average a couple hundred miles and then have to change the plug because it has carbon build up covering the electrode and it runs like crap. I changed it today and it looks like the tip is burnt. Put in another plug and its still miss firing on number 5. So I thought maybe I bought a junk plug and changed it again and its still misfiring. Any thoughts?

 

Compression test will help give a clue of whats going on in that cylinder.