If you notice display problems in Ubuntu such as flickering graphics, freezing display, etc., it is possible that either your machine or graphics card does not have enough CPU or memory to handle the default Ubuntu display settings. The display settings can be changed via the system preferences to try to prevent this from happening.
Go to the System - Preferences - Appearance menu.
Select the Visual Effects tab. On this tab, select "None". This should help alleviate any display issues that are occurring. The amount of CPU or memory to paint graphics on the screen should lower significantly when selecting the "None" option.