Please post full system specs, including the monitor resolution.
A few tuning tips:
Disable or exit all other running programs.
Update the drivers, sound, motherboard and video, but always set a restore point first-You know it makes sense
As a general rule is is better to run the monitor at its native resolution rather than lower the resolution and apply AA.
Lighting effects are hard on the system, turn shadows, reflections, sun rays and smoke effects down or off.
Physics can be hard on the CPU, and PhysX particularly can hurt game performance for certain systems, again, turn it down or disable PhsyX entirely.
Lower the overall detail levals.