PioneerX

Just to close this out for everyone. I finally figured out what is going and it's something I have never come across in my decades of IT experience. 
 
When the system is under load (read/write/parity rebuild) then it's very stable, however once the load comes off it fails. The actual reason turned out to be very obscure. When under load the motherboard fan control would run the fans up to around midway to deal with the increased case temp, however once the load is removed the case temp starts to come down and so the fan controller reduces fan speed, when the temp reduces enough (around 10mins from removal of the load) the fan controller hits it's minimum setting and shorts the fans leads out, this causes the PSU to trigger short circuit protection causing a power outage to the machine. Once the machine comes back online UNRAID detects an unclean shutdown and starts a parity check. The check causes system load which causes temp and therefore fan speed which keeps the system stable until either parity check is cancelled or completes.....then once the load is gone and the fan speed fall off the controller shorts again and the cycle repeats.
 
I have never seen a fan controller short out in this manor before but I have confirmed it with an oscilloscope, the PWN duty cycle drops off as expected but when reaching zero the power and ground pins get shorted together (this is what causes the PSU to fire short circuit protection). I'm guessing the controller IC or the power MOSFET its controlling have gone bad.
 
I will be replacing the MB/CPU/RAM since I cant replace the integrated fan controller and replacing the MB means replacing all the others parts as well.
 
Thanks for everyone's help along this road.