Another data point, this seems to be related to the multiload indicator that shows multiple bar-graphs in the Gnome panel. I switched to this type of system indicator when upgrading to 12.04 (I previously used a different one that showed only a single bar graph, but I could not find that one again after I updated)
Here is a link to the multiload-indicator that seems to be causing the problems:
Another data point, this seems to be related to the multiload indicator that shows multiple bar-graphs in the Gnome panel. I switched to this type of system indicator when upgrading to 12.04 (I previously used a different one that showed only a single bar graph, but I could not find that one again after I updated)
Here is a link to the multiload-indicator that seems to be causing the problems:
http:// www.omgubuntu. co.uk/2012/ 02/easily- monitor- system- resources- in-ubuntu- with-indicator- multiload/
Note that I use this in the gnome-panel (below) rather than that indicator bar that it at the top.
Since I have disabled this multiload- indicator, the problem has not happened once.