I'm experiencing the exact same problem. I have GTX 1080 Ti and everything else runs great. D2 runs great as well, until about 30-60 mins of playtime. I don't think I had any of these issues whilst finishing the main campaign. I don't know if it is a coincedence or not, but they seem to have started after joining a clan, which seems to be the case for a lot of people. Currently up until the stuttering starts the game runs really smoothly and is enjoyable, but as soons as it starts I just quit and possibly come back the next day.