This is probably one of those examples of "if it ain't broke don't fix it".
If an older driver worked fine...and a newer one did not...then it's the fault of the newer driver. If switching back then doesn't work it will be because 'bits' of the newer driver are causing issues.
You'll need to fully remove the ATI card drivers from the system...and re-install the old one....
Or perhaps a restore to prior to the new install....if you have one...
If you need to do this, ATI offers a "Catalyst Uninstaller" which will remove all trace of ATI driver files from the system. You can access it via the downloads > utilities section of their web site.