Watch your CPU temps when you start watching a movie, could be a bad cpu fan. And its over heating. The computer will crash or shut down automatically when certain temperature thresholds are met.
Here are some utilities to use if you cant find where to check the temperature.
Beyond this, also pull and reseat your RAM as this can sometimes cause some wierd issues. But i suspect it's an overheating issue.
https://www.tomshardware.com/how-to/how-to-check-cpu-temp-temperature