i assume both have diffrent timings, you need to find the optimal timings for both to work stable togheter , play with the timings in the bios also try to set em to *optimal* if you got that option and after every change test the stabiltiy with memtest86+ http://www.memtest.org/download/1.65/memtest86+-1.65.iso.zip (i suggest youll let it do at least 11 passes)