My rig:
3.9 GHZ quad core
8 G RAM
single GTX 580 1.5G

I have AA set to low, Game, Object, Particle and Water effects set to high. Everything else is medium. The game defaulted to high on everything, but even with turning some down, I get constant FPS dips to 30fps. It'll be 60 fps for a few seconds, then dip down to 30 fps. It's quite jarring. It even affects gameplay at times, when my character speeds up real quick, then slows down. Is my computer really unable to handle this game?