I just saw that you already posted about this problem last year:
http://www.subsim.com/phpBB/viewtopic.php?t=46785
Silent_otto wrote:
"Were them both the same mark and model? If not, emember that with RAM, issues many times it's not that a chip is faulty, but that 2 or more of them don't "carry on well" between themselves... If they are not exactly the same, many times it's a matter of trial and error before you find 2 chips that work well together..."
He´s right, the problem could be that the two aren´t exactly the same and that they don´t like to work together.