I have a similar issue. I am using a dp to mini dp, not sure which version and my gtx is the 880m. It only goes up to 144hz, although it shows the option 240hz, but when I try to use the 240hz the screen freeze as this image:
https://imgur.com/a/Nzz60X0 . Then the screen come back to the normal as 144 hz. I don't know what is happening, maybe my cable doesn't have a good quality.