About 1.5 years ago I received my Nokia Lumia 925 phone as a replacement for my Nokia Lumia 620, which I had lost. I couldn't be more happy at that moment. It has a great screen, it's quite fast and nice to look at.
The only downside was, it sometimes rebooted while I was doing something. This happened almost all the time when I was taking a picture, but also when listening to podcasts, driving my car, reading e-mail, etc. It's rather annoying when you are using a phone and it reboots almost all the time, especially if you don't know why!
After having read a lot of forums, discussed with some colleagues and tried a lot of stuff myself, I discovered the Wi-Fi Sense (Dutch: Wi-Fi-inzicht). Disabling these options solved the issue of the spontaneous reboots. As I didn't use these options anyway, I'm not missing out on anything either.
So, if you are experiencing (a lot of) random reboots on your Windows Phone 8 phone, try turning of the Wi-Fi Sense options. It might help.