Framerate locked to 30 after some times

This did not happen before the update to .220.

Basically, after playing in the game for a while, the framerate as shown by nvidia drops to 30 fps. This happens especially after I have walked around some highly complex scenes like where there are tons of torches.

To get back to the original 60 fps, I need to press alt-tab. I don't need to go back to the desktop, but simply by pressing them it will be back to normal for a while until it happens again.