if there are mods installed - especially any third party applications that run on the screen, delete them.
had problems with some.
last resort - completely uninstall AC on steam before doing reinstall. worth a try.
Did you upgrade to Windows 10? I have seen several thread with artifacts just like that in the official forums (which is where you should go for technical support) and all of them have been a result of windows 10 with AMD gpu's.