In terms of making the card stay at one speed, it can't be done, it's built into the BIOS that the card will automatically decrease its clock speed as temperatures increase. You have to just find out what the max stable clock is, set that to be the highest clock it'll ever go to, then suck up the fact that you'll lose 50MHz or so as the card heats up. If you try to do it backwards the card will be unstable when you first launch an application and it's cold.
It's worth mentioning than on a 1080Ti the difference between 1950MHz and 2000MHz in terms of FPS performance will be 2.5% at the very worst, annoying but not worth losing sleep over.