The only thing that I can conclude is that its your psu but im not sure off to really assume that. How long has this problem been occurring since you first began gaming ( if that is when it first began).
started about yesterday
just loaded left 4 dead 2 when in game then entire comps shuts of
then i booted it up and i hear beeps then 10 secs later i could boot up
EDIT- now it seems to be running fine
seems like it runs great then bad i think not sure though
hopefully its not my gpu or cpu i dont want to get new parts