I have a Sapphire 6950 and the underclock problem (average low FPS) tested the problem and used MSIafterburner utiilty to OC my CORE and MEMORY clock by only 1MHZ each.
Started the game again and the FPS improved a lot (still stuttering randomly and the "lagginess" feeling) went back to desktop and the charts in the utility showed the correct throttle up when in game of the Mhz and the throttle down when in desktop.
The utility is free and work with every ATI/AMD RADEON card, you can get it from
www.guru3d.com.