Hi and welcome to the Gigabyte Forum.
Well a nice little puzzle.
From your testing logic dictates that if the memory modules aren't showing upo faults on their own that they are ok therefore it must be one or more slots that are at fault. Now that doesn't necessarily mean the motherboard because now, don't forget, the memory controller is built into the CPU, so it could be either. You need to mark all the sticks and then you have some time consuming testing to do.
