StarcraftWire.net relates warnings from Blizzard that the just released version 196.75 drivers have faulty fan control software, causing intermittent framerates from overheating. They are recommending reverting to the previous drivers. Here's word:

We're getting reports where users are getting intermittent low FPS after installing these drivers. It seems that it is related to the fan control included in these drivers not working correctly and is causing the video card to overheat on 3D applications. This will affect Warcraft 3, World of Warcraft and StarCraft 2 Beta. Please uninstall the drivers and revert back to the older ones.

Windows XP: http://www.nvidia.com/object/winxp_196.21_whql.html
Windows XP 64-bit: http://www.nvidia.com/object/winxp64_196.21_whql.html
Windows Vista/7 32-bit: http://www.nvidia.com/object/win7_winvista_32bit_196.21_whql.html
Windows Vista/7 64-bit: http://www.nvidia.com/object/win7_winvista_64bit_196.21_whql.html

More information can be found on threads such as these:
http://forums.nvidia.com/index.php?showtopic=161525
http://forums.nvidia.com/index.php?showtopic=161503