According to a recent report from Germany some Lumia 920 devices from Nokia (this one is on carrier O2) are suffering from a constant rebooting issue. It is yet unknown whether the problem is only with that carrier specific model (or on that service provider), the global unlocked units too, or it also affects the US variant on AT&T also.
However, apparently the issue has something to do with the radio (or its settings). The proposed temporary solution until Nokia acknowledges the problem and fixes it is to go to your settings and disable LTE, setting the highest connection speed to 3G instead of 4G. Let us know if you are having this issue (the type and location of your phone) as well as whether the above managed to solve your problem.