Many people have this problem, we discuss here, https://forums.codemasters.com/topic/20054-dirt-4-pc-stutter-60fps/.
Video about the problem. https://www.youtube.com/watch?v=Iy1nbnsQQZw&feature=youtu.be
I have this problme just with my steering wheel without steering wheel and use keyboard there is no stuttering. (The latest AMD chipset driver from AMD website a bit helped but the problem still persisted.)