Look at the ceramics in the plug. If they have a spotted, metallic appearance, it's detonating. Ask anybody that runs nitrous what causes disappearing plug straps. Too much timing. I'd back it down 2 degrees and try it. Just because it made the most dyno power at a certain timing setting, doesn't mean that's the best when in your car.
Like mentioned, some engines can be sensitive to weather conditions, especially ones tuned on the edge of detonation.
Step up 5 jets... yikes... If it was that lean, it would be surging badly in high gear.
