Demon,
My car developed a stumble/mis-fire/hesitation as well. Pulled the original plugs at 85,000 miles...installed NGKs like the AGSF replacements (extend further). Car ran great. Did nothing else but replace NGKs with the AWSF-32PPs, and reset computer. Day 1...fine. Day 2...little miss/stumble. Day 3...lotsa stumble. When you 1st reset the computer, its run in a rich mode. But as the computer learns, it leans things out. That's when the stumble starts.
Its real...and when that's the only mod done...something's up.
BTW...there is no chance of "physical damage" Good god would have happened the instant you started the motor. Don't you think Ford would have been inundated with broken engines if that were the case?
