Going through a similar issue, posted it here, Shameless plug:
Seems I've narrowed it down to CPU spikes on my i7 8700k, at least that's the most reasonable guess so far. I'm slowly doing more testing too see if there is a correction for the problem.
If you think your CPU utilization is good. I'd suggest making sure your MSI Afterburner Monitoring is set to 100ms and the two updates rates for RivaTuner are also either st 100ms or 0ms. After doing that see if you can get MSI to log a monitoring history.
If microstutters are happening that often you might be able to get a whole match done and see if you get sort of random spikes on CPU usage.
The interesting part about this is the FPS, nor Frametime graphs or monitoring show it happening. No spikes on those just CPU usage.
