Jon,

The P3 is the hottest plug in the range. You might try a P4 or a P6 for hotter weather. Check the top of the piston and the head button, is there any pitting? Pre-detonation will kill plugs like you're describing. The plug wire will also be gray and pitted as well. Check those things first and then go from there. I have other things you can check but it's more involved. You can PM me if the first suggestions don't work out.

Socko