I don't think thats a problem about 390 series. I'm currently using 390x also. Playing Witcher 3 etc. without any problems. But also i have similar problem. Its not happening when i'm playing tho. Its just when i use alt-tab afterwards when i return to game it crashes. Also have a post about it.

http://forum.square-enix.com/ffxiv/t...60#post3252360