Comment 29 for bug 156550

Vikas, /var/log/dpkg.log should give the necessary information. Try degrading to 6.7.197 again, https://edge.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/1:6.7.197-1ubuntu1 and pick your architecture under Builds, then "Resulting Binaries" on the left pane.