Do you have a feature called "Dynamic Overclock" on?
My MSI board has that feature, and I had it set all the way to 10% Overclock (Basically, when you enter a game that requires more power, the system overclocks itself a certian amount.) I would go into CS:S and it would crash, since the board overclocked itself, causing instability.
Turn down the Dynamic Overclock if it is on. It may be called "Automatic Overclock" Not sure what ASUS would call it, but i'm pretty sure it's there, since their boards do support overclocking.