Well, it’s not a problem per se. But I assume it’s just causing the problems you see. Maybe the SD card started to somehow fail some weeks ago. This may cause the problems you’re currently facing: that playing videos stops and that apps don’t react (maybe because of problems accessing data). I mean SD card is not failing completely but neither working perfectly any more.
So you’ll find often here in the forum that using SD cards as “internal memory” is not recommended.
One sample post with reasons can be found here.