Seems like the right section to me. Anyway, we need a bit more info before we can help. Chances are it's a heat issue, but we'll go ahead and get all the info anyway.
Is it an abrupt change in FPS, or does it just slowly decline? What kind of FPS do you normally get? Has it always done this, or did it work properly before?
Finally, to test whether or not it's heat related, you need to find out what temperatures your card is experiencing. It's possible it's getting too hot and throttling itself to prevent damage. I typically use SpeedFan for this, which is free and is just a google search away.
Go ahead and install SpeedFan, or any other software to get your GPU temperatures really, and start gaming. If you can keep the temps up on a second monitor, great. If your GPU stays south of 90C, you're fine. Any higher and it's overheating, and is the source of your problem.