Carlos, isso é um bug do driver da nVidia. Eu tenho aqui uma Geforce4 com a versão 96 (96.43.09-0ubuntu1) do driver (a sua versão deve ser a mesma). Se você sabe ler inglês este é um bom lugar pra ver (inclusive com soluções):
https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-96/+bug/294076Eis a solução que resolveu para mim:
Teclei 'sudo gedit /etc/X11/xorg.conf' no Terminal (Consola) e deu isso:
# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg
Section "Monitor"
Identifier "Configured Monitor"
EndSection
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 24
Option "AddARGBGLXVisuals" "True"
EndSection
Section "Module"
Load "glx"
EndSection
Section "Device"
Identifier "Configured Video Device"
Driver "nvidia"
Option "NoLogo" "True"
EndSection
Então eu adicionei mais uma linha na seção "Device" com: <tab>Option<tab>"RenderAccel"<tab>"False"
salvei, reiniciei o ubuntu e o problema se resolveu no o mesmo com os efeitos ativados.