Comment Best practice (Score 2, Informative) 724
is to swap the memory modules to find out which is causing the problem, if not motherboard.
Also i don't see how memory tests running inside an OS can be effective, i'd much rather boot off of a smaller system on a DVD, USB-stick or floppy to run a memory test.
Dell servers have those Dell Diagnostics CDs that are very small in memory footprint just in order to run diagnostics on memory. But even they're not perfect so you often have to take memory out and see if you can reproduce errors.