PC constantly going off for no reason

Hello, I have a problem with my computer. When I'm using it, sometimes the screen will go black and 'Power saving mode DVI-D' will come up. The computer then stops responding to all commands and my only option is to restart it. Sometimes it fails to even boot and I have no idea where to start looking for a solution.

Here are some details about my computer.

OS: Windows 7 Ultimate (updated with newest service pack)
CPU: Intel i7 3.66gHz
Graphics card: GIGABYTE Radeon HD 5970 (2GB)
RAM: 6GB