Try Gold Memory, from www.goldmemory.cz , because it usually finds
errors faster and takes roughly 40 minutes to test a 512MB module.
www.realworldtech.com rated it better than even MemTest86, and while
it's found errors missed by MemTest86 in some of my modules, MemTest86
also found errors that Gold Memory missed.

Some mobos just aren't reliable with more than two memory modules, at
last not at their standard settings. OTOH lots of modules I've tried
that had no-name chips on them failed testing, even major brand
modules, like Kingston.