I get this same effect, and have posted this issue recently. The only responses have been video card issue, or broken previs / precombines. If you use any mods that affect settlement cleanup such as Scrap Everything, or mods that drastically change cells...they could be the culprit. I will be testing this shortly, I just have to install F4 in another location, which will be easier than disabling all my mods, and going through the .ini files to verify them as vanilla. As far as the video card, you have Nvidia, whereas I have AMD, both being relatively new...doesn't mean they are not failing though, but the odds are probably higher than the previs / precombine issue.