Hi all, had a misfire on my 2001 fiesta zetec s when cold. Expected it to be ignition based, but borrowed a compression tester to check nothing inside was at fault. When I removed the plugs I found this-


Only 2 of the plugs were cracked, with this being the worse one, the other hadn't opened up. On replacing the plugs (to go get some new ones) the car was missing all over the place, and running on 2 cylinders if I gave it anything more than 1/4 throttle. Now, a new set of plugs solved this, but, as the 'old' ones were only about a month old, it's made me worry that something is amiss. Any ideas what's caused the plugs to do this?
Thanks, Sam.