I have not tried Knoppix, just Puppy Linux. It has never frozen on me (yet). But Kubuntu freezes after a bit, whether from an installation or even from the live installation DVD.
I forgot to mention that I have a buddy with some parts coming over tomorrow, so I will also be able to swap out hardware to try and narrow the problem that way, but I figured I could also try posting here to see if there are any other ideas.