it could be a loose or bad cable, but it might be the psu because when a psu goes bad it doesnt mean it wont boot it means it will fail at various points, i had a bad psu that would start to boot windows and then shut down and then it would boot to windows and be fine untill you went to do something that took some power like play a cd and it would turn out. the first point of call is to check all the cables and to replace any you can with others you might have spare.