how about changing the values in HKEY_CURRENT_USER\Software\ATI\ACE\Settings\Runtime\Graphics\OverDrive5, both of them to false?
edit:actually it can be disabled through overdrive page in CCC (that's what I did anyway). by default, overdrive is enabled (the switch is on in the page) even though graphics overdrive (tick box) in the sub-page is disabled. this is my last resort, if the problem still persists, maybe I just settle with alternating between power options (by preventing monitor sleep especially when I'm downloading). by the look of it, it has been quite some time now it hasn't occurred. by the way I'm using DVI to VGA adapter, maybe that's why ctrl-alt-delete and also re-attach cable to the port don't work.