I'm using Windows 7 and I'm not a pro with it so I'm not really sure what my power savings/screen saver settings are on.
I have my computer hooked up to an AOC 19" HDTV/Monitor with the HDMI cable and adapter that came with the vid card. When I leave my system on and walk away for a while (screen saver or power saving features), the screen will end up with a No Signal message. Sometimes when I move my mouse or tap my keyboard to wake it up, the screen won't come back and I have to restart my system to get it to come back.
Try deactivating screensaver and only activating the "monitor turn-off". If it still freezes try the other way around.
It can only be one of 3:
Either it's the screensaver, the "turn monitor off after xx minutes" or the S3/4 (sleep/hibernate) state.
You can change this in power management or whatever it's called in Win 7.