The driver may have failed; but in some cases the error can come from the game itself. Essentially it throttles because it doesn't need full power. Uncommon, but does happen.
What I would do is use your overclock utility; default to factory speeds. The next thing I'd suggest is run this: http://go.microsoft.com/?linkid=9779124
Or follow manual steps:http://support.microsoft.com/kb/978305
Sometimes after the driver fails; it doesn't re-register with your Operating System Key. After you've done those two steps; reboot your machine. Open up your game and it should be running at your factory clock you set.
If it is running at your factory clock; then close and start Overclocking again. As I said when that happens it's usually related to your Overclock causing a fail to your driver. Hope that helps; if you need any more assistance let me know.