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Just for information:
The symptom were short blackouts in the upper part of the monitor.
The problem was the driver from Nvidia version above 470.
Installing the 470 driver from the Devuan repository led to error messages (but worked without flickering or any other problem).
So I got me the NVIDIA-Linux-x86_64-470.199.02.run from Nvidia download site and installed it by changing to tty1, logging in as root and
service slim stop
apt purge *nvidia*
cd (to the directory with the driver)
sh NVIDIA-Linux-x86_64-470.199.02.run
reboot
Done.
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