The workspace between the top and bottom Gnome panels occasionally dims about 25% and holds there for an unpredictable period of time. Sometimes it never recovers to full brightness. The cursor moves but buttons are non-responsive. Most often this happens in Firefox but is not restricted to this one program. The top and bottom Gnome panels remain fully functional.
The processor stays at an approximately steady 40% "User" use. The memory hovers at about a 40% mixture of "User", "Buffers" and "Cached" use. Network and disk activities flatline.
Sometimes the workspace un-dims itself and returns full control. When it does not the only things that work are pressing the automatically presented force quit button, typing "xkill" from a terminal or choosing to restart. Usually the application that dimmed can be restarted without restarting the operating system. I've experienced this in both Karmic and Lucid.
What am I missing?