Thanks for the feedback.
I don’t believe in a power management issue, because sometimes it hangs only seconds after starting. And the issue came up without any changes to hard- or software, the box is usually offline.
And it’s not a real wakeup. As soon as the mouse stops, the image does, too.
On the upside: The rebuild runs stable, it may turn out to have been a good idea anyway. Some things are much quicker/clearer/easier as when I did it first time. Maybe some of my changes to the original code caused it – the features we modified are included in the stock version as of now (toggling contour lines).
For the time being, I think I will stop investigating and invest in recreating.