IMO (I'm certainly no expert)...if the RAM modules are not matched (all the same), there is a chance that timing differences exist which interfere with them all running together.
I had a situation like this earlier this year. I looked at the Memtest results and then fiddled around with the modules until I verified that the timing differences were the key factor. Once that was done, I then went into the BIOS and downclocked the FSB...to see if that would enable compatibility between the different modules (I went from 200 to 166).
When I rebooted with all of the mods in place, the problems had all gone away.
What I learned: Just because the RAM module says that it runs at a given speed...that doesn't make it compatible with different RAM modules of the same stated speed.
Second previous suggestion re Memtest 86+. Memtest86+ - Advanced Memory Diagnostic Tool - http://www.memtest.org/#downiso
AND Icrontic » Diagnose with Memtest86+ - http://icrontic.com/articles/diagnose_with_memtest86
Edited by hamluis, 18 June 2009 - 10:29 AM.