Did you try with only one stick of RAM at a time, testing in each slot? That's what my problem was with Memtest. I had both sticks of ram in there at the same time, and Memtest didn't give me any errors. But once I took out one of the sticks of RAM, errors began to pop up with both sticks. If you've already tried that, then I'm sorry, I don't know how else to help you.