Rather than regurgitation everything here, check out this thread at XDA:
4.2 random reboots - xda-developers
The Reader's Digest condensed version: The only reliable way to stop the reboots is to disable location access in the Nexus 7's settings. Obviously that means that Google Now and Maps won't work, but it's better than rebooting. Google has acknowledged this and is working on a fix.
FWIW, my username is revmra over there so you can see the hoops I've already jumped through to try to nail this down. There are links to the Google bug reports there too.