when I open a video file on Ubuntu 12 - the file is stuck and screen blinks
 

when I open a video file on Ubuntu 12 - the file is stuck and screen blanks off for a second or two then comes back on again for about 10 seconds then repeats for different intervals.
Screen was fine before I tried to open the file.
while researching the issue on the same machine and screen ( toshiba 55" TV via HDMI) the problem miraculously fixed itself - then when I tried opening a video file again the problem came back.
I tried using VLC and banshee.
The machine is intended to be a media center, hence the connection to the TV.
Oh and the 55" monitor is being detected by Ubuntu as a 40" TV.
Fixed the resolution with xrandr, but problem only appears when I try to play video.