I'm pretty sure this also happened to me. I would turn on my N810 and once you got the the menu everything was locked (all the buttons, keyboard, touch screen etc). Then after about five seconds it would reboot on its own. And it would do this non stop until I manually took the battery out. Luckily a reflash solved the problem and i'v never had the same issue again. It sounds like your having a similar problem. Reflash it.