Thanks for putting up a good spec list

There's one more detail I'd like to know: what brand and model of power supply is it using?
If the power supply is a good-quality brand with adequate wattage (I'd start at about 350W for that rig), then the next thing is "what memory voltage is being used?" I'd use 2.7 volts and see what happens, if you're using Auto or 2.5V-2.6V.
If you want to be more scientific about it, download
memtest86 and run that (takes a long time, let it complete at least one full set of tests), then bump the voltage and try again.