I am having this problem from the time. I have installed Ubuntu 12.04. It is installed inside Windows, I don't know if the problem is related to that.
I am experiencing this sudden freezing/hanging, especially when I am using Firefox. When it freezes no Ctrl + Alt + F1 will work and no Ctrl + SysRq works. This is becoming a very bad problem for me.
Please help me solve this. I believe if I keep on directly restarting the computer, my computer may get damaged any time.