> I would boot from an installation USB / CD (preferably the one you set the
> machine up from) and go into Rescue Mode, and see whether the hardware still
> continues behaving the same way.
>
> That means you're testing it with the versions of kernel / modules / boot
> loader / etc as you did when you set it up, without any updates.

Good idea!

I just did that on the latest Devuan Beowulf Live distro (kernel
4.19.0-14), and... no flickering!

Kinda a relief, if you ask me...
Now, where to start to look for the software defect?

Bernard (Beer) Rosset
https://rosset.net/