Most common causes for similar complaints are Graphics Driver
and Adobe Flash.
If you have updated your graphics diver recently (before the problem started) then roll the driver back to the previous version.
You could also try uninstalling and reinstalling Flash.
Always good for us to know what you have for hardware. Your SevenForums profile page has a place to fill that out.
Then we could see if you have a nVidia 500 series graphics card which recently experienced problems with the latest nVidia drivers
and Flash video.