Why are thumbnails & previews not displaying correctly in Bridge CC?

Thumbnails & previews are not displaying at all or show up as blank grey boxes that flicker. If I leave the window and come back to it sometimes the images return, but usually I have to quit and restart. Happens to any files, all sizes, and seems to be random – some thumbnails are fine one time and don't show up the next. I'm using Bridge CC (8.0.1.282) on a Mac Pro, running OS 10.13.5.