Since I updated to 11.04 my Computer seems to restart randomly. The screen goes black, sometimes it comes back If I move the mouse or press a key on the keyboard (but maybe that was just coincidence) but most of the time the screen stays black for about 15 seconds and after that I see the spash screen of my BIOS and the computer is restarting.
It seems to only happen when I am not using the computer directly (manipulating mouse or keyboard). So most of the time I will come back to the computer and it will just present the login-screen or the screen will go black while I am watching a video, listening to music etc.
I am using Ubuntu Classic without effects with the proprietary nVidia drivers.