I also had to stop forcing VSync in the driver and up the max pre-rendered frames above 1 (also in the driver.) However, once I'd got that sorted I can run with all options maxed out at ~50FPS. This seemed to be limiting me to 30FPS artificially and causing the HD problems, so I suspect some kind of interupt storm. However, I eventually realised that MFAA (in the driver) wasn't compatible with TXAA, neither of which apply to AMD stuff. I have a striped 2 disk raid array as well. What was especially interesting for me was to see that HD activity (set up as total activity on the graph) went to 100% at some points and stayed there, especially after a while of driving when it was trying to load in more textures, and then the game started stuttering badly. If you have a second monitor (or even if not) try using HWiNFO to set up some graphs so you can see what's going on a bit better.

It doesn't help that the settings so often need the whole game to restart. I've just spent 10 hours trying to get it to run properly on my nVidia 1060 6GB and Intel i5.

Daz not using gpu