for some weird reason, my computer has been shutting down by itself. whenever i play games, throughout the middle of the game, the game would sometimes freeze, and a jarring sound is made. afterwards the computer shuts down by itself. it reboots automatically, however- there is nothing appearing on the screen. after a few moments the computer shuts down by itself again, and process repeats. During this cycle, there is no display is on the screen. sometimes continuous beeps would be made from the motherboard, which I later found out that it was meant for power error. However i have a 550w PSU. After switching off and turning on the power, the computer boots up fine but may crash at a later time again.
The problem might be that the computer gets too hot. That would explain the freezing and the restarting. The jarring noise could be the fan going into overdrive for a short time.
It being due to heat would also explain why it happens during game play. When playing games your computer has to work harder and produces more heat in the process. As heat takes some time to dissipate it would also explain why there are irregularities to when it will be able to do a full boot again.
A solution could be to try and see if there is a problem with any of the fans. Is there any of them that has stopped turning? See if any of your heat sinks have been blocked by dust and such.
Like bloodoak said, check your temps with speed fan and core temp. They are free apps. You may have an issue with your power supply though. What brand is it? I have seen your issues before and its always been a driver problem. Have you updated the drivers directly from Nvidia?