Weird problem on my 3820

Hello, there. Put it simple, I have a 3820 installed on a sabertooth x79 with a 4x4 g.skill set. At the time I got them, mb constantly reported dimm a1 and a2 were abnormal, I tested everything except on the CPU,and they were all good. For the past 6 months, I just put these 4 rams in slots b and d, running at dual mode. Today I got another same mb from my friend and tested them, weird thing happened on that board, slot c1 can't even detect a ram, slot c2 always reports abnormal, and, the rest 6 slots are working well.
I'm thinking is it the time to drop the conclusion and say the issue is on CPU?