The problem disappeared after the latest updates xorg in oneiric.
Linux ubuntu 3.0-1-generic #2-Ubuntu SMP Wed Jun 15 20:28:47 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux fluxbox_1.3.1~dfsg1-2_amd64.deb xscreensaver_5.14-1ubuntu1_amd64.deb xorg_7.6+7ubuntu1_amd64.deb ...
The problem disappeared after the latest updates xorg in oneiric.
Linux ubuntu 3.0-1-generic #2-Ubuntu SMP Wed Jun 15 20:28:47 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux 1.3.1~dfsg1- 2_amd64. deb 5.14-1ubuntu1_ amd64.deb 6+7ubuntu1_ amd64.deb ...
fluxbox_
xscreensaver_
xorg_7.