I had this problem as well. I see you have an AMD processor... do you have the CPU frequency scaling enabled? And do you also have an NVIDIA graphics device, and are you also using the proprietary driver?
I read somewhere that AMD's frequency scaling causes problems with NVIDIA's driver for some reason.
http://linuxtechie.wordpress.com/200...nvidia-driver/
http://ubuntuforums.org/showthread.php?t=587905
I wish I could say I solved the problem, but instead I just built a new computer because my old one was getting pretty old, anyway. And the freezes haven't happened with the new one, which is also running Gutsy.