I tried everything, I undervolt for -50mV, -30mV, -10mV, 0mV. I tried GPU clock from 1100MHz to 1170MHz, memory clock from 2000MHz to 2150MHz. I lower powertune -5%.
What setup you use? PSU is gold rated but there is possibility that it makes a problem.
pull 1 card
-50mv
gpu clock
1050
memory
1950
see if it runs nonstop with the five cards and the settings above.
if it does run well for 30 hours
keep the lower settings and add the sixth card if it crashes your psu is too weak.
go back to five cards