I was having the exact same issue with my lg g2, so I traded it in for the lg g4. In three days, the g4 did the same restart about five times at random intervals. A few during web browsing, once while driving with gps on, and once while composing a Facebook message. So I took that phone back today to get a replacement. It had been working good, but just restarted on its own a few minutes ago. However, it may have been an update, as it updated one app when it came back on. I'm hoping the issue is solved and I just had some poor luck. One thing to note: every time it has restarted, it doesn't go to the LG screen like a manual reboot would , but goes straight to the at&t screen instead. I'm running out of ideas on what to do if the issues persist. I tried factory resets multiple times without luck.