A while back I had a problem with my computer locking up and becoming completely unresponsive if left to idle for a little while. You should check this thread here. I found that it was caused by my graphics card, and I believed it to be defective. However, upon RMA'ing it and receiving my new one, I find that it too has the exact same problem. I know it isn't my system because my system works perfectly when using a HD 6950 in it. My card also causes the same problem to occur in my other two systems.
My problem: My system stops responding if I leave it to idle for 20min - 1hour. I am forced to power off by holding the switch and restarting it. If I disable AMD FUEL service it works after idle, but comes up to a black screen where I can ctrl+alt+del to get to the lockscreen and continue work.