Screen turns black for a few secs while watching full screen video

update your drivers and also check if your PC is having an adequate power supply to power your graphics card. If you overclock, make sure you don't mess with the PCI-E bus frequency.

Sorry people, it was just a problem with HDMI cable connection. Never thought bad cable connection can cause such specific glitches as it was...

Check your windows power settings? Switch it to Max/High performance and see if that fixes the issue. If it doesn't, try taking the video card out of the motherboard, use a can of air to blow out the cartridge slot it was in, and reseat it firmly. It sounds like a loss of connection or power, but only momentarily.

Also, try downloading a linux live CD such as Ubuntu or Fedora; and running glxtest or some videos in full screen there and see if it also blacks out. That will either confirm or rule out a driver/os issue.