It doesn't completely shut off...but the video loses so many frames that it's unwatchable, and it goes out of sync with the sound.
I don't know that there's a specific reason. But it's beyond aggravating.
If I wasn't so broke...it'd be something of a motivator to get a new machine one of these days. This one isn't ancient, but it's not new anymore, either, by any means.
That's a shame. I was so happy to get my new computer, but I need a decent machine. I hope you can afford a new one sometime relatively soon...