I don't consider it broken so I haven't replaced them, the system is rock solid stable.

Your statement though was interesting, how do you have both ECC memory report errors and 'undetected errors'. At least from a memory perspective, with ECC an 'undetected' error is a multi-bit error that both flips bits and leaves the ECC bits in a legal configuration. That seems like it would be pretty rare.

That said, I've seen motherboards (in our data center) where the memory slots themselves were unreliable (probably bad or weak solder joints on the DIMM sockets or missing terminator resistors). They appeared as a machine with a lot of ECC errors but the same DIMM in another motherboard gave no errors.