With my first 'hybrid core' (that locked up constantly) I'm still not positive what caused me the grief, but I can say that after pulling the nVidia 8400 and replacing it with a nVidia 7300, and running it now as a test Media Director, it works better -- but still not perfect.
That box is being sold (I bought a new AMD64 for the hybrid/core), but I did spend another $100 on the 7300 video card just to satisfy my own curiousity. The video card was definitely one factor.
I've run in under UI2 with and without transparency as a MD. For the most part it seems to work, but when watching TV or a Movie for 90 minutes or more it will slowly start to freeze until it becomes unresponsive and I have to power cycle. There are plenty of fans inside (in fact, it's too damn noisy) and as a straight ubuntu machine it works flawlessly.
There's nothing else in the box at the moment (no tuner card, no extra nic) so I can only assume there's something goofy with the bios and/or motherboard.
In answer to your thread, I solved it by purchasing a completely new box. See the link in my tagline for specs. The sad part is, not everybody can afford to do toss more money after bad into their setups.