Confirming bug as it is reported by several users. Marking Importance as High as this seems to be regression from Dapper for several users with different ATI cards.
It looks like that Xorg's power managements kicks in. Can you try disabling Xorg's PM by adding the following to you /etc/X11/xorg.conf:
Section "ServerFlags"
Option "NoPM"
EndSection
Note: if you already have "ServerFlags" section, just add Option "NoPM" there.
Confirming bug as it is reported by several users. Marking Importance as High as this seems to be regression from Dapper for several users with different ATI cards.
It looks like that Xorg's power managements kicks in. Can you try disabling Xorg's PM by adding the following to you /etc/X11/xorg.conf:
Section "ServerFlags"
Option "NoPM"
EndSection
Note: if you already have "ServerFlags" section, just add Option "NoPM" there.
This may workaround the problem for you.