I have actually seen something similar to that before, although not on the Asrock. In my friend's old comp, one stick of his ram suddenly decides to be incompatible with his mobo. It is pretty weird, as it would work fine on another board, and pass memtest too. There was nothing wrong with the sockets on that mobo too, as I've other sticks work on them. In the end, I just thought that it was dodgy memory, and swapped him with some spare I had. If you just bought that geil ram, return it to the store as doa, and get a new set on the spot, no questions asked.