Solution from:
http://askubuntu.com/questions/468887/ubuntu-14-04-lts-freezing-frequently
Solution:
Your freezing problem may be caused by an issue with your EVGA GTX 570 graphics card. Nothing in your graphical interface responds is often caused by an issue with your graphics processor, for example a missing graphics driver. Unplug the HDMI cable to your Sony television while you are troubleshooting the problem. Then you can reconnect the television after your operating system is working correctly.
Open the Additional Drivers utility (Dash -> Additional Drivers), and select the latest Nvidia driver, version 331 (proprietary, tested). Click the Apply Changes button to install it and reboot the computer to enable it.
Make a note of the superclocked/overclocked settings of your graphics card for backup/restore purposes. Remove the Superclocked/overclocked settings from your graphics card and select its normal settings instead.
