I opened up Netflix on my Nokia Lumia 520 and started to watch a tv show. After a couple of minutes of playing, the app told me that there were media errors/playback errors.
Check if there's an update available to download. Hopefully a fix is included in it.
I will assume pre update everything worked fine. As did min. Now I am lucky if a 10min youtube video will play without issue. Microsoft update has murdered a previously excellent phone.