I had some issues with videos stalling briefly over the weekend, while showing a spinner. I downloaded those instead using the download button under the video. They only took seconds to download and other content was fine, so it wasn't the speed of my connection. Yesterday, all lessons I tried streamed ok..
I do have some problems with some end of video test exams out of sync or not resolving the answers. I haven't tried those in a different browser so can't say whether those are not working or just that I'm hopeless with PHP :D