I can't with two 980Tis; whatever I set the memory to it always throttles down to precisely 3304MHz which is even lower than stock . I've tried all I can think of, Afterburner, PrecisionX, new bios, updated to the most recent xmr-stak but always the same behaviour. I'm due to take the nuclear format option on the OS soon anyway but I would like to know it's not inherent with xmr-stak and 980Tis and is indeed something in my system instead.
I doubt it's specific to the mining software, but you could try running a different cryptonight miner, or one for a different algorithm.
Are you using integrated graphics, or one of the 980tis for display?