Is it possible to try it on a other computer?
I think mostly that the drivers are corrupt installed. (happend me once when I had my 6600GT) It only helped to format the PC.
But in this case, I would first test it on a different computer. If it works then try to remove your drivers and use Ccleaner to completely remove your driver.
EDIT: Also, Are you using Ati 8.9 driver?
Try to enable Manual fan control and raise it up to 60-70%
I think mostly that the drivers are corrupt installed. (happend me once when I had my 6600GT) It only helped to format the PC.
But in this case, I would first test it on a different computer. If it works then try to remove your drivers and use Ccleaner to completely remove your driver.
EDIT: Also, Are you using Ati 8.9 driver?
Try to enable Manual fan control and raise it up to 60-70%