finally03gt

About a year ago the trusty 'stang left my bride to be on the side of the road. A roadside inspection discovered a spark plug had backed out. I reinstalled it, then later that week checked and replaced all of them. Then last week it did it again, on the way home from my honeymoon. Only this time..... no luck putting it back in. It seems the threads are stripped in the head. I found a helicoil kit to repair it (haven't tried it yet), but why does this happen and how can I prevent it. I'm considering putting a touch of high temp silicone on the threads when I reinstall the plugs. Any ideas?