It probably stays at 60 because you have Vertical Sync on. This is good; it's the maximum amount of frames per second that your monitor can display. More than that is pointless. 30 is sad, but it's probably because others don't have AA on (if you don't, you should, but I'm guessing you do).
Duh, forgot about V-Sync. Can't that also play a role in the framerate drop? I remember seeing something that with V-sync will drop the framerate to 30 if the hardware can't make it to 60. Regardless, turn V-Sync off (again, assuming you have it on) and see what that does.