MacBooks are notoriously fussy about the installed RAM. Additionally, these only officially support 8 GB. I know it's possible for them to accept more, but it could definitely be related to your issue.
According to Apple, the officially supported specs for RAM replacement on this model is DDR3 1600 MHz in either 2 GB or 4GB modules. If you're using RAM rated at slower speeds, this may be the issue. It may also be that one of the replacement modules has a minor flaw. You can use memtest86 on a bootable USB to test the RAM sticks just in case.