Back to the topic at hand - doesn’t it seem strange that only CPU4 finds issues in memtest86? It could be a CPU or even motherboard that got damaged and not the DRAM itself, no?

I noticed that, but OP said that he ran the thing in three different systems, so I’m assuming that he’s seen the same problems with multiple CPUs. It may be — I don’t know — that memtest86 doesn’t, at least as he’s running it, necessarily try to hit each byte of memory with each CPU, or at least that the order it does so doesn’t have errors from other CPUs visible.

I also wondered if it might be a 13th or 14th gen Intel CPU, the ones that destroyed themselves over time. But (a) it’s a mobile CPU, and only the desktop CPUs had the problem there, and (b) it’s 11th gen.