I tried to use the 12.4a hotfix, but it didn't work.
I tried to rollback to 12.3 but it also didn't work.
I then tried to rollback to 12.2 ; i downloaded the drivers from here ( http://forums.guru3d.com/showthread.php?t=359696 ), uninstalled all ATI products, rebooted in safe mode and used Driver Sweeper, then I installed Catalyst 12.2 drivers... and it worked ! The light sources aren't pixelated anymore.
Afterwards, i updated from 12.2 to 12.4a hotfix and it still works.
Hope it helps.