Welcome to the forum!
First thing that popped into my mind was that it might be overheating. The experiment that I'd try is to put a fan blowing over the top of it, make sure that it's keeping cool, and then see if it shuts off while you're watching a video. If not, then it's getting too hot. If it is getting too hot, there are ways to fix that (like giving it more space around it for air flow or putting a small muffin fan on it).
Adam