Honestly if I look at your CoD clip it looks fine.
Yes it stutters a few times, but most of that happens in the intro, not the actual gameplay, which is mostly smooth. Your actual gameplay starts at 3:18. The problem/cause could be anywhere from game engine to storage, PSU, OS... Or a sign of components showing age. One thing I don't suspect is GPU. The SSD with your game on it, might have been near max capacity. But this might just as well be a saturated CPU core or a hiccup from background tasks...
The CoD clip has nothing like the consistency of your AC:O stutter, for example. If your games are playable in a general sense, I wouldn't go overboard in replacing components. Take a look at the software side first. Installed / running applications taking up RAM and using CPU should be minimized/eliminated, check storage SMART values and free up SSD space so it has room to breathe; you can do a clean driver and OS install and try different ingame settings.
Also, any stutter that you can reproduce at a specific part of the game is game-related, ignore it. You can play that CoD intro again, if it stutters at the same moments, you can cross it off.
This is not likely a performance issue in general sense. Your spec is fine for normal gameplay. What is your FPS capped at? If you run uncapped FPS, use a cap or use fast sync if you run a high refresh screen. If not, you can try adaptive vsync, the latency it adds isn't too bad. This can help soften the blow of any performance hiccups.