Guys, I keep cracking the porcelain on the #5 spark plug. I've cracked 3 now, and only got 100 miles on the last one. Any idea what is causing this? I'm not beating on the car at all, driving like a grandpa trying to get max economy. Plugs are Delco 41-101. Electrodes are fine, it's just the porcelain. Tried to upload pics to this site, but it says they are too big.