Its possibly power throttling, These cards use 280w+ if over overclocked so if you have 2 of them , maybe your PSU is having a hard time
I have two computers. One has a ftw 980 ti and one has a msi 980 ti. The ftw card doesn't downclock at all during benchwarks and what not. The msi card will downclock at stock settings soon as it hits 61c.
Both cards have the same settings in nvidia control panel. The ftw card is powered by a 750w psu. The msi card is powered by a 860w psu.