I gave up trying to race on Thursday night due to stuttering graphics. It was minor but undriveable. This had just started happening this week - don't know why.
I did some research and someone suggested renaming Assetto Corsa folder in Documents to force AC to recreate that folder. Once I did this the graphics stuttering was fixed. I am also now using PP filters with success which I couldn't do previously. Worth a try for anyone having less than stellar graphics performance.