I have to delete .plist files..and it usually starts working right away. (com.apple.systempreferences.plist and com.apple.preference.desktopscreeneffect.plist) I've tried resetting the PRAM, etc. The problem gets fixed, only to return about a week later. It's so frustrating, my screen saver is always set to turn on after 3 minutes. Help would be great - thanks.