@Mark
Most likely it is a driver bug. Disable desktop effects and check if it works then.
In any case I would report a bug against your graphic driver. You can use "ubuntu-bug xserver-xorg-video-<DRIVERNAME>" to create the report and attach all needed information.
@Mark xorg-video- <DRIVERNAME> " to create the report and attach all needed information.
Most likely it is a driver bug. Disable desktop effects and check if it works then.
In any case I would report a bug against your graphic driver. You can use "ubuntu-bug xserver-