This happened to me too the first day or 2 after I got the OTA update. Usually rebooted when was browsing the web in Chrome. Noticed that the phone would get a bit warm, and then the screen would go black. The everything would be unresponsive for a good 2-3 minutes, then the phone would boot up on its own.
But after those first few days, it has stopped doing that. Not sure what was going on.