U2518D, micro-stutter every 20 seconds or so

I have been experiencing micro-stutters on my high-end gaming PC. Every 20 seconds or so there are two short stutters. I spent 2 weeks tracking down the issue, running every sort of test you can think of. I initially focused my efforts on my operating system believing it was an issue with Windows 10, I ran LatencyMon to track down any DPC latency issues, still no success. The issue persisted and I installed various Linux distro's and the exact same issue persisted across games, video and even when moving my mouse on the Desktop or scrolling i would notice a stutter. Next, I turned to my hardware and went as far as replacing my RAM, GPU, SSD, motherboard and PSU and testing different CPU's. You guessed it, the issue was still there.

At this point I almost gave up and accepted the fact that my £2000 rig was going to micro-stutter every 20 seconds. Then I decided to plug my other PC into the monitor via HDMI and viola, the exact issue was occuring, same thing with DisplayPort. I plugged in my Mac and the same issue occured. I replaced the cables with expensive HDMI and DP cables and no luck. At this point I was 100% certain the monitor was causing the issue.

I am very surprised that by 10 year old monitor can run everything silky smooth and my £300 Dell Ultrasharp monitor can't. I have a few family members and friends who after seeing the clarity of the U2518D are planning on buying one, I've told them to hold off as the monitor clearly has this annoying issue. I don't think i need to post a video of this as I've seen other threads on here demonstrating the issue. Whilst I don't get any tearing with V-sync on (tearing is extremely bad without v-sync), I do have these stutters every 20 seconds or so. Otherwise, everything is silky smooth. I can see why many U2518D owners would not notice this issue as it just seems like a little lag and you would never suspect that it's your monitor unless you go through the stuff that I did.

I did some research and tested my real refresh rate (see here), my refresh rate is calculated at 57.8Hz (see here)



when it is supposed to be 60Hz. This could explain the stutter as after some time an extra few frames must be accounted for. I would appreciate any help and support on this from Dell or other users who have experienced this issue and fixed it somehow.

Thanks.

Setup: 
i7-8700K
Nvidia GTX 1080 Ti Founders Edition (connected through DisplayPort/HDMI, also bypassed GPU and plugged monitor directly into motherboard and issue still occurs)
Windows 10 Home 64-bit
Resolution 2560x1440
Windows power plan - Balanced and High Performance
Monitor purchased from Amazon