Firstly, thank you for taking the time to write back - i appreciate it
I've had some slight success since last night - i decided to give LibreELEC a go, and it plays Youtube videos fine (using the add-on), and other videos from my USB stick - with NO annoying flicker.
The issue is still there with Raspbian though
OS - I'm currently using Raspbian (downloaded from NOOBS_v3_2_0). I had the same issue with my previous RPI4, when using NOOBS_v3_1_1.
PSU - using the official Power Supply for RPI4, from the PiHut website.
config.txt - I've made no changes to this.
Browser - I'm using Chrome, the default browser that came with Raspbian.
Windowed/Fullscreen mode - i get the same flickering issue regardless
Micro HDMI cable - I'm not sure where my original cable came from, but i bought one from Amazon recently https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1
(if LibreELEC plays the videos correctly, then surely it isn't a HDMI cable issue?)
I will try to record a video this evening of the flicker issue.
Thank you very much!