Why is my game stuttering?

In my experience, the most common cause of this (provided the hardware is beefy enough to support the game, which seems to be the case here) is the defragmentation of the drives. I know modern Windows OS versions claim to do this automatically, but there needs to be sufficient free space on the disk in order for this to occur. Personally I find that it's not good enough, and often perform the task manually every few months or so, especially after installing and deleting large files (usually games in my case, some with several thousand files or more).

One other thing to try is to look at your paging (swap) space. Contrary to what some think, the swap space can be too large, or if the paging file is on a heavily fragmented drive, that can cause issues too (unless it was there when the disk was formatted, it shouldn't be fragmented at all).

Hope that helps.