I couldn't say exactly... When I was building my [now ex] girlfriend's computer, I had put two sticks of Kingston 512 in there- exact same ram. When I booted up, it would completely freeze at varying times... I tested it like crazy with memtest and everything.. in the end, it turned out that it would only run properly when I overclocked the system... in fact, everything ran completely stable when overclocked. Something about what the RAM ran at was weird.. not quite compatible with my mobo or cpu or something. My point is this: I just think that the stick of 512 and 256 isn't quite compatible, but almost is... close enough to boot sometimes, and randomly fail other times. So, as for why... I dont have a clue. Not that what I said helps even a little bit :P Actually, i'm really surprized that nobody else has said anything about this, since it's been up here for a couple days at least. Anyways, if somebody else does respond, I expect that they will have something more insightful to say about it. I'm just going off personal experience for this one. Best of luck to you though. Any progress- let me/us know. I'd like to know if you can figure out exactly what it is... Really, im sorry that I couldn't help more though.