This time I was visited by seasonally appropriate graphical artefacts (i.e. small white boxes appearing and disappearing randomly). I was running the system under average to high load when they paid a visit. Switching from proprietary nvidia drivers to "nv"-drivers fixed it slightly, but it quickly turned worse so I rebooted. The artefacts mostly vanished,
Fig: Residual graphical artefacts. The entire screen looked like this, or worse. I tried to screenshot the mess at the time, but at that point, it was so bad I couldn't distinguish any dialogs, and therefore had no way of saving it.
This would indicate that the source of the problem is either the graphics card, or the memory corrupting the nvidia driver.
I'll run some more memtest86, and if that doesn't find anything, do a GPU transplant.