Dougster,
I’m having the exact same problem!!! I have a notebook (new dual-core, 2GB RAM, plenty of GB on HD) and a desktop. Both machines have the same problem. Rendering video-only or an AC3, when the render gets to a certain point, it just stops working!!! Very strange. As with you, this happens when the source was HD.
I would be grateful if you have found a solution yet and would share it with me.
Thank you!
James