I got my ATT one x in November and had no problems until Feb. Swapped out the phone and it also did it. I swapped that one with my current one and it also randomly reboots, but less than the others. It does this while in my pocket so I thought the on button might be getting held down for an extended time. If I do this manually it will cause a reboot. Hold your button down to try it - it might be your problem. I altered my OEM case to protect the on button somewhat and this seems to have helped. Strangely all 3 phones also got hot sometimes, but not always, about the time it would reboot to make me feel it also may be a battery problem. Temp would reach 110 F + and this was usually while it was not being used, in my pocket or not.