Mac Mini Video Output Over HDMI Stops

I have a Mac Mini, previous one to the current version, running Yosemite which is being used with a Samsung TV as a home theater PC. About once a week when I go to use it no video will come through to the TV over HDMI. The input is fine and all other inputs work. I then VNC into the Mini and it is always responding fine. When I go to display settings it shows unknown display but that is the only option. The only way I can get video output back is by rebooting the Mini. As soon as it reboots I see the Apple logo and it starts fine. Any ideas what this could be?