If it is fine when one memory is out, I would say that the memory has the problem.
Try putting in just the one that causes the error and leave the memory checker running for about 24 hours, or long enough for you to be certain.
A lot of people use memtest or similar, but then only run for 5 minutes - whilst this will show you really bad modules, it does not show the modules that just have slight faults.
I wrote my answer simply by saying in my opinion, the most common problem. As others have said, it could also be the motherboard itself.
I have to say that it is very rare as problems with regards to the socket usually prevents the machine from turning on and power problems usually give some sort of BIOS alert next time you turn on - however, it does happen and should be mentioned just incase.
Once you know that a stick of memory is fine, you can try swapping it with the stick that you think has the errors. If it is still showing errors, it is the stick - however, if it is fine then put the working stick in the other socket.
If after this you are still getting errors, it could be a faulty motherboard.
At this point, if you have done any power tweaking in the BIOS, undo it or return the BIOS to safe settings. If you haven't done anything then I think the motherboard has problems.