You can get memtest to tell you where the errors occur. Run memtest once and write down those numbers (hopefully, it should only be a few different locations). Then shut the computer down and run it again. Make sure that the locations are the same. If they're not, I'd tend to suspect the motherboard.

I have a friend who's having this exact problem right now, and the locations of his errors are different every time, and the memory works fine in another computer.

Of course, I'm no expert here.