I've had a X7859 for a little more than 2 years and was pretty happy with it until it started randomly rebooting a few months ago. At first it was occasional but now it's more 5-10x a day and it's clearly interfering with my work as a freelancer (software engineering).
It happens on my main OS Windows 10 but also on Ubuntu. It's not linked to the temperatures as I've been monitoring them and am employing a cooling pad. I don't have any software/OS errors and as it's happening on both OSes I think it's more a hardware issue.