I know what you're talking about and it's iPlayer's fault. I don't know what causes it though. Sometimes when I copy over tons of video files (usually anime), about one file every ten files, it just ends up
loading nonstop.
Delete the file, create another folder and copy it again there. If it loads nonstop again, then delete and create another folder and copy it over again. Eventually it will work.
I know it's not an issue with broken clusters or anything like that, since I've tested the microsd card throughout and it hasn't reported any faults.