So I don't understand how other people can have amazing FPS on high settings. If that was the issue wouldn't it be same across the board?

Ok so I just upgraded my comp to a GTX 560 TI along with a 600w power supply. I've also got 8GB of RAM and an AMD Phenom Quad Core processor. My computer should be able handle the graphics easily on high, but I'm still seeing low fps in pvp, flashpoints, and big operation groups. In pvp the fps usually sits at around 20 to 25, but consistantly drops to 9 to 15 when a lot happens on screen. So I'm seeing chugging when I really shouldn't be. Keep in mind I'm running the graphics at the bare minimum, everything on low and shadows off. If anyone has any incite into this I'd appreciate it. I just don't know whats up with the computer.