so is it the undervolting that is killing the cards?
I remember I ran my 290x (single card, just as a hobby) almost 24/7 at 80 C for months and month with no issues. But 100% STOCK.
prolly a combination of Overclock and undervolt causing to many amps to run thru the cards. Just a guess If your looking for longterm Overclock is not your friend
Best Regards
d57heinz