I even use other psu just to supply mb,cpu,ssd..so wierd..im out of ideas 
whether it always happens with the same GPU card, same riser or the same PCI-e slot on the motherboard? Also, did you follow temperature and power consumption at the moment of GPU off?
Did you try to add power to the 5th card from another PSU, or to use the second PSU reserved for MBD, SSD, CPU...