Could be a software problem, something you did install, or Windows is corrupted, drivers are bad, etc. I have the same machine, with 3 instances of a game started (Archeage), temp is constantly at 85-86C and it only reboots when I update the video driver or Windows. For me, this machine is very stable.
Please check if you ever update the windows, like the graphic driver.
If the updated driver doesn't match with the system, it will cause problem.
Please roll the driver back to the original one if you did update it before.