I have the same problem, it occurs almost around every 2 days or less, so what I did was, usually I apply a custom theme, so I changed it to stock, it lasted for 1 week, and after that, it still reboots. So what I did next was going to setting>about phone>system update> (the three dots on the top right) > Download Latest Package
Wait for the package to download and apply the update

And now it sould be fixed