Mac Mini drops HDMI signal, can't screen share in, have to reboot, help!

Setup:

MacMini (Mid 2010, 10.6.8) -> HDMI out to Onkyo Receiver -> HDMI out to Sharp HDTV

My MacMini has started exhibiting some strange behaviour if I leave it running over night. Typically I will shut my receiver and TV down before I go to sleep, but leave my mac mini on to run updates, downloads or backups. When I turn on the receiver and tv the next day, my Receiver reads "No Signal" coming from the Mac Mini. The light on the mac mini is on and steady, so it's not asleep and hard drive's connected to the mac mini have their lights on and steady as well. The Onkyo receiver and TV work fine with everything else that's connected via HDMI (PS3/Xbox/Digital Cable Box), so I don't believe they are the culprits.

I also have a Macbook 4,1 running 10.6.8 that I've tried using Screen Sharing to access the Mac Mini. Screen Sharing just hangs after accepting my login credentials, no window pops up, but if I click on the drop down menu "Window" the computer I'm trying to access is there with a check box next to it.

I've tried disconnecting the HDMI cord from the Mac Mini and reconnecting it, but nothing changes.

In the end I resort to hard rebooting the Mac Mini, potentially interrupting anything happening at the time. Once it boots up everything is fine... until night comes and I fall asleep...