Viewport artifacts

[trashofmasters]

Hi all,
I have random artifacts on some elements of the viewport.
Images are better than thousands words:
http://i28.tinypic.com/25rmvd5.png
http://i26.tinypic.com/2rw28pf.png
Ubuntu 7.10 gusty gibbon
Gnome 2.20.1

Does anybody know why these elements looks so strange?
Randomly also opening windows (during the animation that occurs just before the application is ready) artifacts appears.

Thanks in advance.