hey jon are you running the extra head shim?? sounds like a classic case of over compression.Try adding the supplied additional head shim and that should fix it,if you are over compressed.
The plug is right i have run many v-specs from stock to JP Modified to the Speed version.I have run P3's in every single engine,i tried many other plugs but the P3 ran the best in every condition.
hope i helped , let me know if it fixes your problem