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.
Open the terminal and enter the following code to remove Gnome Screensaver
#sudo apt-get remove gnome-screensaver
Open Administration -> System Monitor and find the process “gnome-screensaver” and it
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
Open Preferences- > Screensaver,when prompted to start xscreensaver daemon, allow it