I occasionally experience something similar. But in my case, it is not an HDMI issue. It is strictly an issue with WMC.
Next time this happens, try just moving your mouse around. Or hit Ctrl-Alt-Del on your keyboard. If the video comes back, then you just need to kill WMC.
The way I deal with this is to map one of the soft keys on my Harmony to Alt-F4. Just kill WMC with Alt-F4 and bring WMC back up with the Menu button.