I'm left to guess it's some web page video called FLASH. Let's not discuss that but please try an old fix. Type this into a web search and use the method you like.
DISABLE FLASH HARDWARE ACCELERATION.
That cures so many web video playbacks that it's not funny. Some games use FLASH for in game movies that it could help there as well.
To go any further I'd like to hear about your systems power supply, make and model as well as exactly how you installed Windows.
About a year ago my PC that I built started to get blue screens and then random reboots. This had a ASUS AMD AM3 motherboard with integrated graphics (no card needed). 3 months ago I decided to switch motherboards after frustration with blue screens, reboots, and countless hours with phone support not working out. So I bought a ASrock board with a AM3+ and transferred all hardware to new board. I also had to purchase a separate video card as well. Thought that this would solve my PC issues but again the PC will randomly reboot or if I am playing games or video it will sometimes reboot. After speaking with more techs they said to replace the power supply so I did but it still reboots. I also gave my video card a stress test which pass with flying colors. I also had a tech look at the errors in a windows 7 screen that I never seen before and nothing really noticeable was there. So now I am thinking its my CPU but all the techs I have spoken with doubt that's the issue.
Anyone have suggestions what to try. I can't spend much money on a repair tech so rather do it myself.