Thanks for looking - here's the skinny:
Can't get 8 sticks to pass prime 95 at 2400, although each stick has passed prime 95 stable for 24+ hours (ran 4 for 24 hours then ran the other 4, all pass). I ran them 24 hours on Memtest, then 24 hours prime 95 - works fine with 4 sticks but not all 8 (8 passes Memtest, but not Prime 95).
Rampage IV Extreme (3301 Bios)
3960x CPU (set to Auto multiplier)
8x8gb kit of G Skill F3-19200CL10Q2-64GBZHD (supposed to run at 2400)
2x680GTX in SLI
2xOCZ Vector SATAIII in Raid 0
3xWD Red Sata II in Raid 5
Using XMP profile 1 (and Mode 1 inside DRAM timings per G Skill's website recommendation), configured with LLC to medium, 180%, the memory is all at extreme, 140% per Raja's guide - but no overclocking yet - just want to pass Prime95 at stock speeds and DDR-2400 with all 8 sticks.
As far as voltages, everything was set to auto except VCCSA, which was set to 1.25 per the XMP profile. The VTT was up at 1.35 and the Vcore at 1.36 at full load (again, these are auto set, those are just the readings).
I can't get it to pass 24 hours of Prime95. I've tested 4 of the sticks at a time and the system is prime95 stable at 24+ hours, so I know the RAM is good.
With all 8 sticks in at 2400, I left Memtest running over the weekend and it made it through 6 complete passes (57 hours) with no errors, but Prime95 fails after about 3 hours. I've upped the VCore incrementally to 1.45 and saw no improvement - first core fails within 2 to 3 hours, and another core about 3 hours after that. I've had VTT as low as 1.1 and as high as 1.4 and it also appears to have no effect on Prime95 stability.
The memory is supposed to run at 2400 - the processor has no problem handling 4 sticks of it, and for at least a few hours all 8. I also know all 8 sticks are good.
I'm stumped. Any help is appreciated.