I recently upgraded my PNY 9800GT to an EVGA GTS 250. I play a lot of source games and used to get 150 FPS or so on my 9800GT. When I first used the GTS 250 I was getting close to 200 FPS. For some reason now, I am getting around 20 FPS. I have no idea what is happening. Any ideas?
First I'd check the temperatures of your GPU, it may be overheating. Secondly I'd check the case for any dust which may be restricting airflow, causing the GPU to overheat. Lastly, I'd uninstall all nVidia drivers, completely remove them then install the newest ones available.