Did you ever get this sorted? I am interested because I just replaced my Mac Mini -> Sony Bravia VGA cable with a Mac Mini -> AVR2310 DVI/HDMI cable. It all seems to work great but then after half an hour or so the screen goes blank like there is no signal. I am not using Windows, just OSX 10.5 and Safari and it just suddenly goes. Sometimes for a few seconds and sometimes for good - everything needs rebooting.
Nope, the issue persists. I even uninstalled and reinstalled windows/boot camp and no go. :-(
Of course, on the Mac side, all works fine (no 30m issue like yours).