No worries, it's a shame we couldn't find a definitive answer.
I hope the rma goes smoothly - let me know OcUK's findings, it's an intriguing problem (granted a very annoying one for you).
Additional: One last throw of the dice - you could run memtest to check your memory's stability. You could even try the system with just one stick (rotate sticks in the first slot if necessary) just to see if by some small quirk of fate it makes a difference.
However, the upshot of the above test is, that even if you found a fault in memtest or the system shut down with one stick, you would still need to rma - so it would be a test purely for the curiosity.