hang-the-9 :
Does it do that ONLY in online videos? Or does this happen anytime you stress the system like with a game? Download and run the benchmark in Prime95, that will run though the CPU and RAM to see if things are stable when they heat up. If it passes that, install 3DMark and run though that, you may need to install the older version of the program if you have a lower end laptop. Run that. If that is also stable, try using a different browser.
Although from the sound of things, your laptop is locking up due to heat or video card issues.
@hang-the-9. To be more specific, when i run online videos such as youtube and movie hosting websites such as novamov, my laptop will just freeze. However, when i went to such websites in Safe mode, it worked perfectly fine.
I also tried different browsers in normal mode. But my laptop still freezes