It also heppens with me as well! (64Bit)

Guys, i have the exact same issue!

CPU: Intel Core i7-980x (with or without overclock same results)
AMD: HD 6990 4Gb (Single card)

About 40 FPS with ULTRA and then after 20 minutes drops down to 15-20 FPS as well.

I wasnt looking at CPU or GPU usage at the time, but i did try to overclock Both CPU and GPU and i get the exact same problem.

Plus, changing video settings in game, it doesn't work what so ever. LOW preset and HIGH preset i get the same FPS issue.

I CANT PLAY WITH THAT ISSUE.

Really hoping they fix that too, since i loved the game.

Same problem here, Intel 980x@4.0Ghz, ATI 6990 4Gb. With Ultra high starts with > 50 Fps and after 30 minutes, i get between 15-20 fps, cant play with that.