Without knowing anymore specifics, I'd guess to say that's its a heat related issue. Especially since it's summertime now and you're on a laptop. Note that when viewing online movies, especially if you choose hi-def over std-def, your laptop will use much more horsepower and thus create more heat than if you were just cruising the internet or working on an Office application. And when viewing a movie, you're doing this consistently for an extended amount of time. Eventually, your computer reaches a temp(usually specified in your computer's Bios) where it will either shutdown or restart in order to cool-off.
To verify and understand this, please perform the following steps:
1) Close any open programs, especially any internet browsers, and make sure your antivirus isn't performing a full scan.
2) Right-click on the taskbar (or hit Ctrl-Alt-Delete) and select 'Task Manager'
3) Now click on the 'Performance' tab.
4) Take note of how much 'CPU Usage' you're using. For example, I'm on a laptop, at idle, and it says 3% CPU Usage.
5) Now, open up your browser and go to where you watch online movies but don't start any videos yet.
6) Take note of the 'CPU Usage' now. Currently, I'm on Hulu and on average I'm at 8% CPU usage.
7) Now, start watching your online movie and take note of the 'CPU Usage'. For me, it jumps between 40-50% on std definition and 50-60% on hi-def.
So what's the solution if it's heat related, well for a laptop your best bet is to buy a laptop cooler (look for reviews online). I recommend any retail store or buying online from NewEgg, Amazon, or Buy.com. Also make sure any vents from the laptop aren't obstructed.
Now to keep a better eye on your laptop's temperature, I recommend the following 100% free, no spyware, no viruses, no adware programs:
(Note: after you install one of these, you may want to repeat the steps above and take note of the temps during each step)
CS Fire Monitor (CPU Usage tray display, etc)http://www.softpedia.com/get/System/System-Info/CS-Fire-Monitor.shtml