Wait, so you had Memtest results that showed the RAM was showing errors in your rig, and they refused to replace it? What RAM company was this, so that I can avoid them.
Correct. I had 2 sets of RAM, each showing errors in both of 2 rigs. RMA on the RAM was refused by the retailer. The manufacturer did offer to RMA the RAM, but I would have to send it to the factory in Taiwan.
I couldn't be bothered, and I'd already spent $10 to ship it to the retailer and paid $30 in "failed RMA" fees. So I decided to cut my losses, and just ditched the RAM in the trash.
The retailer did, with their refusal and bill for testing, send a screenshot of memtest showing it had completed several passes successfully. They did test the RAM on a different motherboard from a different vendor.
The RAM showed intermittent errors - not like genuinely faulty RAM. Typically, you would get a flurry of errors, once every 2 or 3 passes of Memtest. The errors were consistent with an address line error - i.e. the wrong memory location was being read, rather than the much more common dead memory cell, hence most of the memtest tests would never detect them.
My guess is that there was a subtle incompatibility between the RAM and the motherboard. Perhaps the traces on the mobo were a bit longer than officially specified, and this particular brand of RAM had slightly weaker input buffers than officially specified. The combination led to occasional failure.