This seem an known issue with MBP with nVidia 330 or nVidia 650 (MBP 6,1, 6,2 or 9,2). What machine are you using?
Could you please try to remove the screen-saver settings file? The file should be located ~/Library/Preferences/ByHosts/ folder and named something like com.apple.screensaver.xxxx.plist. Set your screen saver preferences once again and see if the issue disappear.
|