Unfortunately, it does sound like you're going to have to drain the loop and check the CPU. It sounds as though you might be dropping a DIMM due to improper mounting pressure or pad contact.
You may also want to check the stand-off screws are not overtightened surrounding the CPU and memory array.
Remove the CPU block and CPU and check the CPU pad pins, then remount the CPU and block whilst checking for even mounting pressure.
13900KS / 8200MT CAS36 / ROG APEX Z790 / ROG TUF RTX 4090