The issue has been reported by HTC 8X users as well
It looks like some Nokia Lumia 920 users in Germany have experienced random reboots when using the smartphone.The issue seems to affect some HTC 8X users as well, which might suggest this is a software mishap that might be fixed via an update.
The folks over at WPArea.de report that the rebooting issue might be solved by setting the phone on 3G network and not LTE, which seems to be causing the problem.
Apparently, Nokia is aware of this problem and is preparing a fix for it, which will be delivered as a software update.
If you are one of the unfortunate owners of a Lumia 920 that constantly reboots itself, you might want to switch to 3G and pray the temporary solution will work. Stay tuned for more updates on the matter.