So your saying both sticks of memory work fine if used alone?
Maybe you should have mentioned that in your post. Did you have 512 and try to add another 512, or did you have 2-512's from the start and just start having trouble with one of them? You have to give us some information in order to help you, I cannot read minds.
Do they both work by themselves in different slots, or do both work fine in only one slot but not the other?