Finder repeatedly can't see files and folders in High Sierra

I did a clean install of High Sierra in Dec 2017 on a formatted late-2016 MBPr. Things were smooth for several months. The past few weeks, the Finder and (sometimes) applications seems to repeatedly lose the ability to see files and folders. I will open a folder, such as Documents, and see the usual folders I have there. But, if I select any of the folders, they appear empty. Sometimes, if I try to open the file from MS Word or other app, or attach a file using Safari or Chrome, those apps WILL see the files/folders, and other times they will not. The problem is always resolved with a restart of the computer. But, this is obviously annoying and disruptive. Why is this happening and how do I prevent it in the future?