(Oh my god watching that clip even for less than a minute has made me dumber, i can feel it)
I believe that this is not an issue with the codec, but rather with the server not sending the data in full. This is not a problem and normally works as intended, your browser will ask for more parts as the video is playing. For whatever reason there is a slowdown getting some chunks.
This maybe due to the internal caching of the server. Let's say that most people wisely stop watching this video after a few seconds, then he server caches the beginning of the video, but does not cache the later parts, which need to be loaded from some slower storage. For me the video stopped playing when i let it run from 3 minute mark and it stopped at 3:10, but then after i have a couple of times rewound it back, it played through that portion with no problem.
As the user of the site, it is unlikely that you will be able to do anything about this.