70% should be okay

set clocks

to

tdp 70%
core +10
ram + 100
fan 75%

see how that works

[/quote]

I use these parameters, except have the fan on auto. It looked like it was running at about 57%. Fired up five of the cards yesterday at 10:30 AM and ran stable until I went to bed at 10 PM. However, sometime in the night it stopped again. I am mining zcash and using the EWBF CUDA Zcash Miner. In the mining apps window it was continuously trying to restart. I close the window, and tried to restart the minor but it would only flash for a split second and disappear. When I restarted the rig, I pulled up MSI afterburner and all my settings were gone again. So after resetting, I have fired up the five cards again with the  fans set at 75%.

Is it okay for the cards to run the fans at 75% for a long period of time?

Not sure where to go from here. I have tried reinstalling the miner software as well as a refresh copy of Windows 10. Could it still be a hardware issue, even though it ran for so long? still haven't tried another PSU on it.
[/quote]

75% fans are fine

I always set manual fans as they tend to stay put.

dropping settings overnight  could mean :
a bad card
a bad riser
clocks too  high   70 % tdp  +10 core + 100 ram is not too high
do the manual fan setting for 75 over night

 if it fails again

run one  card less   then you are running now see if that works

if you have 1 bad card or a bad riser  on a big rig  say a 6 to 10 card rig  it will take time  to find the problem.