Have you downloaded and run a memory testing utility like Memtest86+ ? It's available here:http://www.memtest.org/#downiso
Download and create the appropriate bootable media and test each memory module separately.
Run a complete test cycle on each memory module.
If the memory module is fine then you should see no errors show up during the test cycle.
If an error shows up during testing then you have a defective or incompatible memory module and it should be replaced.