Sometimes the video playback will stop and the error may seem that the video has not fully loaded.
This "bug" seems to happen if the playback position meets the "video loading" in the time-line.
In the time-line, the left "brighter" area is the video which has been played. The medium shade area is the video which has been loaded. And the darker area is the video which has not loaded.
Avoid having the Play Point position cross over (or get too close to) the Not-Loaded Video area. If this happens, the video may stop loading and you will thing there is an error.
To have the video area fit to your computer screen, click the screen icon in the bottom right corner of the interface. This is at the very right end of the play-bar.
This will not show the play-bar, nor the section markers on the left. But will allow you to see the full screen of the video.
Browser Cache Size
Full loading problems may have to do with the Firefox. Raise cache size to 1024 (the max allowed.)
Firefox Support Site
Thanks to Andy Rosen for debugging this issue on Windows.