My files are in Dropbox and many are stored in online-only mode. When I highlight a file that is online-only, it starts to fetch it from Dropbox and while that is happening, DOpus can be slow or unresponsive. Awhile ago I was experiencing full DOpus crashes as a result (Preview Pane instability with Dropbox's online-only files - #13 by ByronWillis)
The only things that I want to have pop up in the viewer pane for my workflow are PDF's and image files. I found that by turning off the .docx and the .xlsx files, the instability issues abated.