CHG90 FPS problems

[Totally Not Woojin]

So yesterday I bought a chg90 monitor as I found a deal on it. Setting it up was no problem, but when I finally tried to play some games. The FPS was constantly changing. From 100fps to 40fps, then 60, then 100, etc. I tried an FPS cap on 60, still problems. I do have free sync on and am running a gtx 1080ti. This problem can be seen on both intensive games and non intensive games. Heck, even Fortnite has the problem. Also, after a minute on the desktop, the cursor and response seems to get higher. Also, another problem I have is that whenever I run a game, it blacks out, then reappears with it saying DisplayPort. It’s on all games that I have. I’ll try to post a video of it happening when I get home. Is this normal or not? The cables are running free and are not stressed. (CHG90 is the 49 inch monitor from Samsung. Not the 1440p version.)