The memory that came back after RMA was the same as the memory already in the PC. If the make, speed, voltage and size are all matched, what else is there that can go wrong. 2 years on, it's possible to buy 8GB, 12GB or 16GB together in one package, but back when I made my PC the largest set within a reasonable price range was 2 x 2GB, so I bought 2 of 2 x 2GB. Even, then it is possible for the manufacturer to use different chips in each batch, but if the chips confirm to the spec, then it should make no difference.
Testing the DIMMs seperately will prove that each one works, but you still have to test them all together. To avoid the risk of damage, I tested them all together first and they passed, well at least the first two passes of MEMTEST. After all, they've been RMA'd, they should work.
With the 8GB installed, I tried entering the QFlash Utility, I got a black blank screen. Removed the upper 4GB of RAM and the QFlash utility worked. Upgraded to the latest version of the BIOS.
OK, put upper 4GB of RAM back in and BIOS passes memory test without getting stuck. Memtest is currently running, perhaps an all nighter will reveal more.
Running memory test now.