.mp4 video files not usable, but here's a solution
We continue to see spinning ball delays and usually complete freezes in every Screenflow project (v7.3) that contains any .mp4 video file.
The fastest and surest solution we've found to completely eliminating the crashes is to open each .mp4 file in QuickTime, then File: Export as..., choose 720 or 1080, and this will convert the file to a .mov.
Not sure what's so cool about .mov file clips but they instantly become usable, scrub-able, fast, stable, and make all the projects work again flawlessly.
Many of the .mp4 video files (news clips, quotes, etc) come from YouTube, Vimeo, Facebook, etc. For YouTube clips, the best app is the free ClipGrab, which does offer different Formats, such as Original (which is almost always mp4), WMV, and OGG Theora. I've yet to test out the latter two, since this .mov trick works so smoothly.
Just reporting in, in case others have similar work flows, and are wondering how to get around .mp4 madness.