turret

Help! My computer screen goes black, the sound goes jerky and then stops and the fan runs really fast. This is apparently random, I have run checkdisk and there are no errors. It happens when computer is cold so is not overheating. It was happening a few weeks ago and I figured it was my 4 year old processor or my mother board so I replaced both and got a new power supply but now it is happening again. When it happened before my hard disk was corrupted so I removed everything from it, formatted it and reinstalled windows, but this time my hard disk is fine. The only bits left of my old computer are my hard disk, my graphics card (which is only a year old) and the wires. I don't know what to do.