Adobe Flash 11 will always run with 'hardware acceleration' option enabled, even if you unticked the 'hardware acceleration' option. Seems good, right? Flash based games will run very fast since it uses hardware acceleration! No. Not good for desktop games. ATI 5xxx will run Flash 11 content in UVD (unified video decoder), and, behold, this UVD will trigger the 2D clock (400MHz) as well! This trigger is implemented in card BIOS, not driver, so no driver update will fix this problem, unless you're flashing your card's BIOS (and flushed card warranty as well). Whose fault? AMD or Adobe? Source: Rock, Papers, Shotgun Forum.
Use MSI Afterburner and save a profile for your GPU default clock or even overclock and then set hotkey to change it on the fly anytime. Or you can disable PowerPlay...
