I have a feeling your card is throttling because of your high overclock. Throttling occurs when your temps are too high for the GPU. When that happens, your performance will be significantly reduced. Here's a few things you can try out:
1) Download RTHDRIBL and run it for 1/2 to an hour. Open up your control panel and see what temps you're getting. Then try running RTHDRIBL on stock clocks and see if your FPS goes up
2) DL 3DMark 05 + 06 and get your scores. Then try running it at STOCK clocks. Compare the scores. If the second scores on stock speeds are faster, then your card is throttling which means your temps are too high.
3a) The most plausible alternative to improve your temps for your video card is to by an aftermarket cooler from companies like Arctic Cooling, Zalman, etc. Mounting one of those should significantly reduce your temps and thus, end the throttling of your card. There're not too expensive, as a good cooler should cost around $30-40 and are relatively easy to install.
3b) What kind of case do you have? You can try installing more fans inside your case to help airflow.
3c) If you have the money and patience and experience, you can try watercooling.
4) If all the above fails, maybe your PSU is not giving you GPU enough juice. What is your PSU
Best of luck fixing your problem.