I had the same issue....
On Wed, my phone randomly shut down and i had to soft reset to start it back up
On Thurs, my phone rebooted 4 times (3 times without a SIM card in, so I know it's not poor reception)
I am using a SIM adaptor (coming from an iPhone 5), so I thought that could be the issue (I was also having issues with dropped calls in areas where my iPhone 5 had no issues) but the reboots with no SIM card take that out of the equation.
I also thought it could be an app that I recently installed, so I uninstalled everything I added since Wed, but I still had reboots.
Given that every reboot/freeze I had was when I was not actively using the phone. I'd guess that the rebooting is related to managing applications running in the background. I'm very surprised to see this happen given that earlier builds of WP8 (and the Lumia 920 in particular) had lots of reports of phones rebooting and I thought that was fixed.
I'm on the fence about exchanging or just returning and going back to my iPhone 5 and waiting to see if Nokia/Microsoft fix this issue (as they did with the 920) I think the pros of a Lumia with WP8 outweigh the cons of the OS (especially since most of the OS related complaints I have will most likely be solved by WP8.1) , the camera on the Lumia 1020 is stunningly good and iOS is just boring to use anymore. That being said, I've owned every iPhone and I've never had one randomly reboot.
Added to that, reading some of the responses here from more veteran WP8 users doesn't give me a good feeling. A phone rebooting (even every 2 months) or not being able to use Here maps (which is one the reasons I decided I could make the jump to WP8 and not lose the navigation options I have with iOS) doesn't sound like something I want to stick with.
That being said, I've owned alot of phones and I usually have no tolerance for instability at the OS level. So the fact that I'm on the fence about exchanging the Lumia 1020 speaks to the quality of the camera and how much fun WP8 is to use (with the value added Nokia software)