For what it's like, compared to my previous comment #5, tonight (same machine,
same distro/stack) I was able to trigger the bug pretty frequently by using the
Epiphany browser with a particular website—twice within the span of fifteen
minutes or so.

So while there is a simple time component (ex: crashes while the computer isn't
doing anything in particular), it can also sometimes be triggered by stressing
the graphic card a little with some operations (such as can be seen on some
browsers).