I have been getting really low FPS with my GTX 980 TI, when I first getting into the game I am getting 60+ FPS but after a minute or so it drops down to around 20 to 30 FPS. I try with and without DX 11 and it’s the same either way with or without having the setting set on max.
This is the only game that I been getting this kind of a problem in.

CPU; I7 2600, 3.4 ghz
Memory 6 GB
Windows 7