I had the 820 and I had several freeze ups but no reboots. I switched back to Verizon and picked up the 822 and so for a very stable device no issues at all. I find the battery life to also be superior to the 820. It could be something with GSM devices causing those issues. I know the 8x on all carriers is having a major reboot issue.
I disagree that it's a WP8 issue; I've now had 3 WP8 devices (a Lumia 920, an HTC 8X, and a Lumia 820), and while I had some issues with the 920 (loss of connectivity, overheating) I've not had a single reboot or freeze with any of these devices. If you have a phone which is rebooting, take it back. Get a different unit.
Mine wouldn't sync my windows live account, so I did factory reset to try again, now it's stuck on the Nokia logo. Ive tried soft and hard reset multiple times. And it just goes back to the nokia logo and stays there, I have it on the charge cable and its just been like that for an hour now.