Before we get too crazy here.....
Are you showing an SES code? It is common because of the duration of the XP cam for the computer to show what it believes to be misfires in the cylinders...this doesn't necessarily mean that you are actually getting misfires. Dave has discovered recently that when you delete the P0300 code it makes it extremely difficult to diagnose an actual misfire.
If the NGK's are working for you, and you're not seeing knock at wide open, then you may be OK. If you are showing significant knock, then stepping back down a heat range to a traditional plug will also help.
