Sorry to be asking for so much information but it does help to get a better understanding of the problem.
Are you using IE or something else: firefox, chrome or opera? Any add-ons?
Desktop PC or laptop?
Wireless internet connection or LAN cable?
If it is a Desktop PC could be the something like a loose internal component (like a fan on the heatsink) or a video card that has worked its way loose. Computer spins up and crashes.
If you are streaming on a wireless card it could be a driver issue:
Computer freezes during Youtube videos? - Internet Applications - Applications