I'm just going to guess this is a windows machine as you said it was also a gaming machine.

If that's the case have you checked the power plan settings in the Control Panel (win 7)?  This happened on a notebook with a mobility 5850 in that the OS would downclock the chip.  Def apples to oranges but the desktop ver of Win 7 should still have the Power Plan settings that can be adjusted.

Under the Power Plan settings set to High Performance and then click the Advanced link and make sure the ATI Graphics Power Settings are set to Maximize Performance.

I also set the screen saver to none, not blank but none.