This is somewhat similar to this post Netflix playback lag and mouse stutters.
I've tried the suggestion to enable GPU scaling which had no effect.
These tests were done on desktop with this site open on firefox and also just desktop nothing opened.
I have 3 displays
1 DP 1920x1080 main monitor
1 DVI 1920x1080
1 HDMI 1920x1080 this is connected to a Sony receiver which switches from Cable TV to computer output(extended display).
all with native cabling.
RX 580 on win10pro x64 1709 fully updated and Adrenaline v18.4.1.
Other info when hovering my avatar.
So what I've found (and IIRC has been happening for the last few drivers and this is debatable until I go back to 17.12.1) is
1) single screen (DP) no problems
2) ADD Two Extended screens ON, no problem
3) If the HDMI and/or DVI screen is powered off then I get this stutter every 4 seconds which lasts for just a bit less than a second. It's affecting my typing right now and mouse movements and if I run a screenblanker it shows it there too. It's at it's worst with both screens off and extended enabled. I use window's "project" to enable extended mode and on a typical day only use one of the extended screens the other remains off.
It doesn't matter if the Sony receiver is on or off, It will pass the extended display info either way.
To summarize. Extended screen(s) when powered on there's no problem, If either or both screen are powered off then I get this stutter.
So I DDU'd 18.4.1 and installed 17.12.1 and extended screens are working fine without the stutters.
I guess that rules out my receiver being an issue and can only be software driver. Will report.