This issue usually occurs after a block transfer, or after moving around town rapidly. This leads me to believe there's some kind of memory or connection leak with the asset loading requests.
What happens is after above mentioned, all textures cease loading, UIs and NPCs will fail to load their appropriate screens (Which will softlock the client with the loading circle of death.) And zoning out of town will work, but then you will be unable to make any action other than basic movement.
During this time chat will be laggy, but still functional. So you can be cognizant of your own client disconnect status.
Occurring on Win10 latest updates, latest GTX 1080 drivers, on both standalone steam launch, and through tweaker.