Although I rarely find the system log/event viewer to be helpful, maybe you can check Event Viewer the next time you have a lockup to see if there might be something reported.
Also, are the 3x 1TB Seagate drives in RAID, or just separate disks? And have you verified that they are running on the latest available firmware?
Also also, maybe I misses it, but do you have the CPU and memory running even slightly overclocked? I know you said Memtest passed, but just had to check.
Finally, as roadworker suggested, if there is any potential for the GPU to be part of the problem (though you say it happens at random times, not exclusively when working with video), be sure to check it.