Thomas has a OnePlus One phone, and he's having problems with it rebooting on its own. What can he do? He can't contact the company. Leo says that it's an operating system issue, chiefly the firmware. OnePlus One is a Chinese company that doesn't offer support outside of China. So the best Thomas can do is replace the firmware.
The chatroom says that if he turns off NFC, the rebooting will stop. That lends to the idea that there's a radio issue with it. Thomas should also try doing a factory reset and loading one app at a time to see if there's a conflict with an app.
XDA Developers has a section devoted to the OnePlus One with alternative firmware options. But if it's a hardware issue, they won't help in the long run. And sadly, cellphone companies won't really help unless the phone was purchased from the carrier. It's a very strong argument for choosing an iPhone. They support their hardware.