isn't the 2cd Ram bank always active when running in dual channel mode (don't know if this the case here or if I'm right)
a memtest might run several hours.
bios update is done in ~10 minutes (plus a test run to see if it fixes the problem).
intel/asus releases bios update not just for fun !
why I'm saying this:
well read up the linked docs at the Intel microcode release page [1] for your CPU and realize that they contain serveral bugs.
some are fixed via microcode and I bet some are fixed with microcode in combination of bios updates
[1]
Releases · intel/Intel-Linux-Processor-Microcode-Data-Files
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