I have the same issue, most noticeable around the plains of whiterun but also around riften when going through the trees. Although I have not optimised my textures so that could be due to mipmaps etc.
I experience all 4 types of popping MW listed in exteriors. Ugrids 7 helped but I went back to ugrids 5 a while back. Although I haven't actually played for the past month!
Yes I have them stuck in flight sometimes even if walk up to them.
But also other animals sometimes have delayed reactions, as in frozen until I get closer.
Also turn off autosaves - that solved those hang ups for me and removed the drop to 2-3 fps on transition to interiors (still get them occasionally on transition to exteriors)
You could try Mod Organiser to track mods (which is what i use, and download) as you can manually enter the version number if for example you do not want the latest version.
Only workaround is that the mods need to be 'installed' to be tracked - which I do by doing a manual install of one single file.