I have the same issue. It happens from time to time, usually when I’m typing something. It doesn’t reboot when it’s in my pocket. It could be a lot of things, never taken the time with logcat or something to test out why this happens. I do run the FP monitoring agent. So if they really monitor it, they should see the error. Clean and new SIM by the way. Others have reported this as well. No explanation from FP support.