You stated that you ran Memtest86....perhaps you ought to read what Wiki says about Memtest86, Memtest86 - Wikipedia, the free encyclopedia - http://en.wikipedia.org/wiki/Memtest
. Attention should be focused on Known Problems.
I would suggest running Memtest86+, in the varying combinations of RAM that you spoke of.
You don't say if the RAM modules are the same speed, type, etc.
I've experienced incompatible modules of RAM which were stated to be the same speed...but running Memtest86+ and playing with bus speeds...proved otherwise.