It's definitely your GPU.
It used to happen to me back when I had a 9600 GSO 512 and even an 8800 GTS 512.
an fps drop is normal whenever there is a high detail. the glow around the enemies is high detail so the fps drops
there is nothing wrong with your graphics card or your graphic plug in settings. if you can play the rest of the game at normal speed then all is ok
i often get fps drops but the speed goes up again