@imx: I do have a Flex mini but it's not between the server and a AP.
The server, HDHomeRun and the AP are connected to a UniFi 16 ports PoE (1st gen.)
Thank you for your help.
When my network ran on TP-Link and Netgear, everything was flawless.
Foolishly, swapped it all out for Unifi - G2 Pro, G2 Pro POE, Flex and Flex Mini. Worst decision I ever made, honestly. Even a x.x.1 firmware release, poorly documented, huge differences/variance between them, even 'things' that weren't supposed to change.
I hate to say it, but I'd put my money on the problem being Unifi something somewhere - the amount of time I've spent running iPerf, PerfSonar, etc, to narrow down firmware issues is scary. Finally, I now have no issues at all... but I have probably gone through 20+ firmwares.
The Flex Minis consistently drop UDP for me - caused havoc with my Sat>IP box, Facetime would stall. Swapped it for a regular Flex, no issues.
That is most likely the issue. TCP between the server and client, but UDP with tuner and client. Wifi and UDP is a bad combo. There is no way to resend missing packets, like there is with TCP. This is the cause of your pixelating/macro blocking.
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Thank you both. I've learnt something today.
@imx: I would have said that I never any issues with my network gear but I never run in all the tests you made, so I believe you. I can say I have some issues with video conference apps (FaceTime, Skype, Jitsi) sometimes and I always thought that was caused by a weak signal at the place I was trying to place the call.
https://community.ui.com/releases/UAP-USW-Firmware-5-43-24/acc26457-b842-4d78-ab56-58e482346835
The above firmware on the AC Pro, is the only one since 4.0.80 (ALL 4.3.x releases, many earlier 5.43.x) that does not cause me any issues on wifi, be that UDP, throughput issues with VLAN tagging, multicast, you name it.
The below on switches is a 'goodun' for me as well. Although granted our hardware is slightly different.
https://community.ui.com/releases/UAP-USW-Firmware-5-43-23/ec50ac66-97cc-4cb7-b619-8cf3f71ba72f
Flex Mini's I've put back in the cupboard 
Thanks for all the info. Very useful.