The fluctuations in boost clock are very minor (13mhz is 1 step I believe), but then that allows the temp to subside enough to then regain that 13mhz. On the Titan X I would just set fan to 100% and wear a headset if I wanted to run it with a fairly aggressive overclock (1400+) and for it to be able to maintain that clockspeed.
I think part of the issue with the fluctuations was to do with secondary components on the cards with reference design such as VRM's getting very hot also, even if the core was fine. I tested this with a couple of Titan X EVGA ACX 2.0 coolers, which do better in terms of cooling the VRM and power section and saw it did not fluctuate as the reference cooler did given similar core temps.