Honestly you would be surprised how even having the wrong RAM timings / speed or incorrect PCIe settings can mess with game stuttering.

For example I have a 3930K processor. It's a 6 core CPU, Sandy Bridge-E so it's getting on a bit. Stock speed is 3.2GHz with a 3.8GHz single core turbo however I have it set to turbo all cores to that speed using a motherboard BIOS setting.

Games on this CPU do not stutter for me in any title. But if I run my RAM out of spec for example it ships at 9-9-9-24 or something like that, if I run it at with 11's instead of 9's I get stuttering in games.

If I run that RAM at 1600MHz (it's 1866MHz memory btw) I get stuttering. If I run the CPU with a BCLK above 100Mhz I get stuttering.

And if I enable the PCIe 3.0 on my board using the NVIDIA enabler, guess what? I get stuttering.

But if I use PCIe 2.0, correct RAM timings.. everything's golden, smooth as can be.

I'm not trying to discount your experience with the 1800X - No doubt a high end 8 core CPU like that can handle multithreaded games no problem (as I'd expect it to) but I do want to say that perhaps your previous system was broken.

Modern gaming is very sensitive to subtle system changes. Being able to do all the calculations the GPU does in the milliseconds they have.. it needs the rest of the system to be perfect if all the cogs are to mesh perfectly so to speak.