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Are you sure they are the same speed? if not then maybe you should check every stick to have same specs in order to work correctly, even tough some memory will work togheter when they have different speed (at single channel and slowest speed of the both) some will not work or will cause further problems .
If you don't know the speed of your memory and the label don't mention it, try to download cpu-z and on the memory tab it will give you the specs you need, do that with every stick and match them. Good Luck!
