Dirty windows with Workplace Switcher

[dld]

I am running Mint 11 with Gnome panels and Workplace Switchers in an upper and a lower panel. When I change workplaces via the Switcher, I often find portions of the windows in the original workplace filling windows in the new workplace. I can force a redraw by moving a window slightly, or by using Ctrl-Alt-arrow to move to a different workplace and then back. Switching with Ctrl-Alt-Arrow gives clean windows. Strange? Is this a Switcher problem? What can I do about it?