You don't mention what card you have, only that it is ATI.
I had similar, but not exactly the same problems in a previous install of Ubuntu and overcame it to a degree by running at 16 bit colour instead of 24 bit, which improved things enormously.
Try the following as your xorg.conf to see if it helps, but backup your current version, just in case.
Code:
# 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 "Device"
Option "EnableDepthMoves" "True"
Option "EnablePageFlip" "True"
Option "DMAForXv" "True"
Option "AccelDFS" "True"
Option "ColorTiling" "True"
Option "RenderAccel" "True"
Option "VGAAccess" "True"
Option "AccelMethod" "UXA"
Option "DRI" "True"
Option "MigrationHeuristics" "greedy"
Option "TripleBuffer" "True"
Option "EXAOptimizeMigration" "true"
Option "EXANoComposite" "No"
Option "BackingStore" "true"
Option "AGPMode" "8"
Identifier "Configured Video Device"
Driver "radeon"
VendorName "ATI Technologies Inc"
BoardName "RV280 AP [Radeon 9200 PRO]"
EndSection
Section "Monitor"
Identifier "Configured Monitor"
HorizSync 30-80
VertRefresh 50-75
EndSection
Section "Module"
Load "dri2"
Load "dri"
Load "dbe"
Load "extmod"
Load "glx"
EndSection
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 16
SubSection "Display"
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
EndSection
Section "Extensions"
Option "RENDER" "Enable"
Option "Composite" "True"
Option "XVideo" "Enable"
Option "XINERAMA" "False"
EndSection
Section "DRI"
Mode 0666
EndSection
Edit or comment out any lines in red above that need to be changed for your system, and if it does not work just restore your own xorg.conf.
Good luck!
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