main cause of this problem- heat
as stated previously, check your temperates either in the bios or using monitoring software such as everest. what you could do to try and rule out the theory that its your hard drive is leave the pc in the bios for 30mins+ and see if it turns off
that way you know its nothing software/side or hard drive related. Again try what other have suggested previously try re seating CPU heatsink and fan and try re applying thermal paste. You might also want to make sure north bridge and GPU have good enough cooling and air flow around the case
let me know hows it goes