I've got an unrooted, plain vanilla 4.2.1 Nexus 4. Sometimes it randomly reboots itself while using it (and for all I know when it's not using it). Less than once a day problably, but I can't find a cause. IT does a weird sort of half-reboot too; it goes immediately to the big glowing X screen without the white Google screen that comes up during a cold reboot (off, then on).
The reboots don't seem to be caused by anything in particular; I've had one while running Chrome and one while running Gameboid. both of those run fine 99% of the time too, nothing has triggered the reboot more than once so I don't think it's a specific app.
Is this a known bug or caused by something or what? I'm mostly only using apps that worked on my last phone (which wasn't android 4.2) and stock apps, so I don't think it's any fishy apps.