high chance PSU.
if u just doing pure surfing and youtube and it doesn't restart then most likely its PSU not giving enuff power to gfx hence reboot.
yea when im just surfing net or watch show on pc it never happens. only when im playing game for some time then it happen. i guess PSU may be the main reason for this failure. will get the PSU check it out when CNY is over (next wed perhaps)