It's happened now in four different projects, and I can't figure out why. If you'd like a video example,
I can't seem to figure out why, despite searching on the forums as well as google. The artifacts are more random when in the preview window (they don't always show up at the same times or places on the same frame), but obviously are always the same in the rendered versions. It seems as though it may be linked to the Sony Levels plugin, when I turn it off it seems like they disappear. Please let me know if anyone has any advice.
EDIT: So, I've found that disabling my GPU (Options>Preferences>Video>GPU acceleration of video processing: OFF) fixed this. If anyone else has this issue, this seems to be a solution!
Not a "real" GPU, an onboard graphics chipset for the Intel Core i7 5600-U processor, it's the Intel HD Graphics 5500. Not sure when the last driver update was. This is my machine, the ThinkPad Yoga 12.