Have you got S.M.A.R.T enabled in your bios
it might be an early warning that your hard drive is packing up.
also if you want to go inside your pc, check there are no loose cards/memory. is the cpu fan working properly (overheating can cause shutdowns) unlikely because you said about lost file fragments. worth checking though.
if you do go into your pc, remember to earth yourself by touching the metal casing of the pc (pc plugged in but wall socket turned OFF)and take care when handling memory, don't touch the chips.