I still use an old nVidia 7800gt with the latest drivers and i've seen that effect maybe 2 or 3 times in my 5 years of playing. It occures on clear days and nites and will "fix" itself when the weather changes or a save and restart. Always wondered about that but its so rare at zenith and self-correcting, that I've never bothered to inquire about it. I run GWX 3.0 too.