I must say that is what I first thought when you mentioned the second kit not playing nice together. And since I wrote the article I have a bit of insight about what was meant by that statement. When Memtest86+ was developed there were not as many matched kits as there are today. So often folks would just buy two sticks of ram, loose sticks. Now today that transfers to our use of mated pairs or kits. You are correct in thinking that there might just be a conflict between your two sets of ram slots. But I am not sure that is the whole story.
First up, I assume that your ram is made up of one kit. And as such it would matter little if you mixed or matched the ram the key issue is when they play together there is a problem.
Remember the CPU's we are using have the memory controller built in. So it is more likely that it will be a combination of a couple of things. First the fact that four sticks of ram are being used at the same time. This adds heaps more stress onto the IMC of the processor. Second will be the addition of more capacity, not as stressful as more sticks but still adding to the loud. And then there might just be a weak slot in one or other of the memory slots.
You did do individual slot testing with a working stick of ram I seem to recall. So what I think is needed is that you start fine tuning your ram settings and voltages to ensure all 4 sticks work together.
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