this worked for me....
http://www.worldstart.com/why-does-my-computer-freeze-when-i-play-youtube-videos/
The video did not simply "freeze." The entire computer froze solid (the cursor wouldn't move) and it rebooted on its own, twice. The first time was with the html5 player. The second time was with Flash.
A computer that suddenly stalls and reboots is not a good thing to have.