Download a bootable diagnostic to a USB flash drive. Let it run for a while. Does your PC shutdown? yes -- power problems, shorts, overheating, failed components. no -- maybe its a software problem. Example: http://www.ultimatebootcd.com/
Try and let us know.
All operation i do in UBCD were successful but after i reboot a few hours later, it still shutdown suddenly when loading the starting screen or loading the beginning screen ( i dont know, i know when pc on, it show manufacturer logo)