no i havn't uninstall catalyst 12.4.just removed the old gpu and inserted new one..then i installed catalyst 12.8
This is most likely the cause of your problem.
You're going to want to go to.
Start>control panel>uninstall a program>click on Amd catalyst install manager>click uninstall ALL amd sofware>follow the steps>reboot>re-install 12.8
If this doesn't fix it, then it may be your card. But i HIGHLY doubt it if you didn't un-install your previous drivers.