I'm also having a similar issue. I just got my N7 a couple days ago and after the first day I noticed this. If I hit the power button to set it to sleep and don't wake it up within 5-10 minutes, I'm not able to wake it up at all. Sleep/wake functions works just fine if I don't let it sit for too long. After 15 minutes or so I can press the key but nothing happens. I've tried changing the sleep times, holding a light to it just in case the auto brightness is going crazy, and plugging it in to see if I get a charge notification, but nothing works. The only solution I've found is to hold down the power key to force it to restart and that has been causing it to freeze at the Google X screen, forcing me to reboot yet again. I haven't seen my problem happen to anyone else yet so I haven't found any solutions. I've ruled out the battery since it does it fully charged as well.
I'm using a 16gb N7 with the latest version of Jellybean and its not even a week old.
Any suggestions or advice would be awesome.
I could just be missing something obvious as I've only had it a short time and haven't checked everything yet.