Beginning with driver version 355.82, Nvidia has added a new feature called "Debug Mode", which will downclock any factory overclocked graphics card to Nvidia reference clock speeds. This is a great feature to help determine problems with the card, or for games that are sensitive to overclocked video cards.

To turn on Debug Mode, open the Nvidia Control Panel, then go to the Help menu, and check the Debug Mode option.
Note: If you have a reference model card, this option will be grayed out.