I feel like performance varies greatly from PC to PC. I also have an i5 2500k OC to 4,5 GHz, a gtx 970 and 8GB RAM, and I struggle to run the game at a smooth 30 fps on medium with some settings even set to low (shadows for example). Mostly I get some super noticable slowdowns every now and then and maybe even a few freezes here and there.
So I just reinstalled the newest Nvidia driver and that seems to have solved all my problems. Now running the game at high settings with stable 60 fps.