Frequent Computer crashes: I've got no idea why...

Every time i am playing a game longer than a certain time my PC crashs. My monitor becomes black like the pc is shut down and the last sound fragment gets repeated very fast for some time. Fans are still runnig untill i press the off-button for some time. When i restart the system afterwards, it just says that the pc got unexpected shut down. I couldnt find an Error in the Windows-log-file-thingy and all the temperatures seemed fine to me.

For my system: Operating System:Win7 32-Bit Home Mainboard:MSI 790FX-GD70 Processor:AMD Phenom II X4 955 Black Edition Graphic Card:ATI Radeon HD 6850