Almost sounds like either a thermal (heat) issue or a flaky power supply ( just a guess ).
If your video card is bugging out (fan -if it has one- has died, overheating etc) it might provide such a symptom as well. You might
Start by verifying your power management settings. Place the theme into "Always On" and see if the problem persists.
You might also boot the system into BIOS and let it remain there overnight as some means to see if it reboots on its own outside of Windows. Not much heat generated while idle in BIOS, so - in theory - it might more point to an intermittent power supply issue.
My guess - just that - is that your video card might be leaving the station.