I've also had this happen if one of the analog ports got hit with a small surge. In my case, it blew the little chip used for that port, and the blown chip caused a short, thus repeated rebooting. Once I removed the chip, it booted up fine. Dead giveaway is when you touch the chip, it's as hot as the sun (impressive for such a small chip).
It was a pain tracking down the replacement chip, but we were able to find it and replace it with success.
Hope this helps...
Kris G.