[camelreef]

I am starting to think that something else on your system (running next to kodi) causes your issue by either blocking the CPU or even causing interference - DVB card? Some server process acting up?

Well, the box is used for 3 things: Kodi, Roon server (well-behave in my experience, but I will give it a go with the service turned off) and Samba. I've cleaned-up a few other things that were running by default and knew I would not need (lxcf, snapd...).

I did a test with Roon server stopped. Same result, artefacts after 30-40mn.

Now doing a test with the movie file on the SSD, ext4 FS, and the external USB3 drive unmounted.