Random Standby Mode!

Hi there - new to these forums.

I have posted this in the Graphics Cards section since I think that is the issue and I could not see a more appropriate section.

First off my specs:
Asus EN7950GT Gfx Card
Asus P5W DH Deluxe Motherboard
2gb ddr2 667Mhz Ram
Intel Core2 Duo 6600 @ 2.4Ghz
Windows XP Service Pack 3
450w Power Supply

For a few weeks now my screen will randomly go into stanby mode (is the green light goes orange and it displays "going into standby mode") 99% of the time this occurs when I am playing games, nothing fancy just World of Warcraft, Frets on Fire and especially TrackMania. A few times though it has happened when just browsing the web or folders etc. But hardly ever. If i rapidly shake the mouse or mash some keys it sometimes comes out of stanby and I can continue playing or what ever I was doing, however most of the time I have to restart my system. Prior to this I have had weird issues where my screen would be filled with multi color blocks like an 80's arcade game gone wrong or the display would be fuzzy or low resolution on bootup, however that is passed just this standby mode issue. I have always assumed its the graphics card or power supply, since then I have replaced the power supply and it still happens. I need to just check what you guys think before I go out and buy a new graphics card since they arent the cheapest of things  

Just to clarify, it happens randomly, I have formatted and reinstalled windows xp and the newest drivers many many times, I have tried with a different monitor with the same problem still occuring, I have taken off the side cover and put a desktop fan blowing straight into the pc - thinking it was a heat issue - same problem. So in my opinion it is the graphics card, as I said I just need some clarification or some more things to rule out other causes.

Thanks so much,
Ross