Try the ATI/binary driver.
Instruction from Ubuntu Here
By default Ubuntu will use the open source ATI or Radeon driver for cards manufactured by AMD/ATI. Some users however prefer the proprietary fglrx driver for various reasons.
The instructions on this page will tell you how to use this driver. If you encounter bugs with these closed-source drivers, developers will not be willing or even able to assist you in resolving your issues. Use them at your own risk. We encourage our users to prefer open source drivers.