Maybe it would be quicker to swap the RAM into a different machine.
I had a PC crashing which turned out to be, I think, a RAM module
returning some bits correct and some wrong, and somehow the EFI test
reported that the RAM was okay. However, a program on SystemRescueCD
and available separately, named Memtest, Memtest86, or MemTest86+ -
I think the third is a fork of the second - found the bad module.
It was designed to report single bad bytes or bits in memory (and then
you just set your PC to not use those memory locations!), so 2 GB
playing up all at once was quite indigestible.
I also once, long ago, helped a colleague at work with their home
computer, and with the boss's consent, by swapping their memory
modules with those in a workplace PC. Since both computers worked
afterwards, the boss was happy to let the memory stay swapped.
The specification of the memory modules was the same, so I do not know
why there was a problem, but the other interesting features, as far as
I remember, were that (similarly?) the operation of cache memory meant
that the BIOS test did not detect a fault, and/or data in RAM was
corrupted if you left it several seconds between writing and reading,
so I suppose you say that it was not refreshing right - that could be
why it passed tests, it had not got corrupted yet.
This was back around the days when well-informed burglars would break
into offices and pull memory modules out of desktop PCs - they just
took the RAM. I accept that this is a while ago, but, even so, you are
using a well-specified PC for a job that I believe could be done by
a Raspberry Pi.