Lag/Stuttering during gameplay and main screen

i just purchased BL: TPS for my iMac and my specs are above the minimum however there is a serious lag/stuttering problem whenever i turn quickly or a lot of things move quickly on the screen. Especially in Concordia. BL2 runs perfectly. What can i do to get this game to run smoother? Here is my specs if it helps answer my question:

3.06 GHz Intel Core 2 Duo
4 GB 1067 MHz DDR3
ATI Radeon HD 4670 256 MB

I know its not very impressive, but still should be running the game better than i am seeing.

As for the game itself, I absolutely love it so far!