Spot on regarding older 4:3 resolutions.. I've recently repaired some older monitors.. so I think that it came from there.

I see why you are thinking of screen rewrite/refresh problems but it's not this causing my particular problems.
This is most easily disproved by the fact that there are no problems at all in either Slackware or Devuan on the same hardware using the same drivers, kernel versions and Xorg setup.

The nearest example that I've found online is this: https://streamable.com/adxdn (not my video). Mine is actually worse as I get intermittent "vertical shuttering" as well.

I can eliminate these problems by using an earlier kernel, but this brings other problems.. chipset support is not as good, leading to poorer wifi and audio performance. Overall CPU performance is worse for Sky Lake etc.
There are kernel options that help slightly (not totally), most of these affect CPU frequency scaling. This was implemented by Intel for very good reasons.. I want to use this facility for the same reasons.

Re: video players. For me, SMplayer is flawless on this laptop using direct rendering in Slackware and Devuan. It is perfect with any video that I've thrown at it..(full screen with every video spec & type that I've got.. and that is a lot!) and I'm **very** fussy regarding video playback performance.

IMHO it's **not** a video performance problem.. The CPU is a SOC.. I'm still having nightmares in remembering the problems of getting a stable Linux installation on Bay Trail (SOC) tablets. I've now started looking at how the kernel handles firmware bugs and it's interaction with Xorg.

Mucho further investigation required...

Bodge99

You're on Intel graphics? i915 support is/was disabled on some kernels, around 4.15.x. It could be as simple as that.
See Here And This Also. I'd check a few logs to see what's going on. I think you set "i915_alpha_support=1" or maybe "i915.preliminary_hw_support=1" depending on the kernel. Sucky older graphics (Like mine) seem to be OK.

I disagree that you must have good video simply because 2 other OSes have. The kernel versions may be different. Distros are capable of doing strange things if need be.

Yep, done all these tests... and more..

I'm currently running kernel version 4.20.3 on Mint, Slackware and Devuan (compiled from the same config) so it's not that.
Xorg (et. al.) are also identical so it's something else specifically within Mint.

It's definitely running the i915 driver.

Bodge99

Ok. I suggest you drop Mint from the list and solve your problem that way.
I have mint in a VM here, and as a standalone OS, and it's not showing any video issues I have noticed in the standalone OS. The VM has vboxvideo, and suffers. I'm also on the i915 module, and have i965 X driver & vaapi drivers (which only vlc uses) and vulkan sdk.