Is it the banding across the top of the halftrack when you zoom in? Looks as though there are "waves" of smoke moving across the surface of the halftrack?

No.

I'm referring to a significant drop in frame rate at that scene.

When playing other parts of that scene, I get 50-60 frames per second with all the detail settings cranked.

When I am looking at that smoke, I get about 3 frames per second on my old card and about 10-12 frames per second on my new card.

Are you getting a drop in frame rate there?