I just wanted to chime in here and give an update. I thought that the issue stemmed particularly from online-only image files, but now that I have turned off previews for .docx and .xlsx (Word and Excel) files, the instability problem has seemed to evaporate!
For those of you out there that don't particularly need to see previews of these file types and are suffering from Dropbox "online-only" instability, I'd recommend this as a workaround-