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Lately I have been facing some issues using Ubuntu 11.04, one of them is the freeze which occurs once a while when screensaver is on. I tried all sorts of tweak in gnome power manager but there was no use. I then replaced Gnome-Screensaver with xscreensaver and finally it worked.
Here is how I did it to fix . Just follow this simple steps and hope it fixes your issues.

Step 1
Open the terminal and enter the following code to remove Gnome Screensaver
 #sudo apt-get remove gnome-screensaver

Step 2
Open Administration -> System Monitor and find the process “gnome-screensaver” and  it

Step 3
Install xscreensaver, type the following in the terminal
#sudo apt-get install xscreensaver rss-glx xscreensaver-gl xscreensaver-data xscreensaver-gl-extra xscreensaver-data-extra

Step 4
Open Preferences- > Screensaver,when prompted to start xscreensaver daemon, allow it

REFERENCES
http://www.multimediaboom.com/fix-screensaver-freezes-in-ubuntu-11-04/