Comment # 29 on bug 917411 from macias -

> Try blacklist the audio
> driver (snd-hda-intel) and test without audio for a while whether
> you get the same problem.

I can do only on weekends, but sure, I will do. What I should execute to
disable it, and then to enable it. 

> > 01:00.0 VGA compatible controller: NVIDIA Corporation G96
> > [GeForce 9400 GT] (rev a1)
>
> So it's Nvidia.  Are you using nouveau driver, or nvidia
> binary-only driver? In either way, it's too bad for debugging. 
> It's almost impossible to debug these (especially without logs).

Ok, but we are forgetting, I didn't change hardware or video setup. For several
months this video card with the driver was OK (audio card didn't change as
well, but the setup might have changed).

I was/am using nouveau driver, nvidia is nowhere on the list.

> > How to turn back dmesg to its default level? I.e. I know how to
> > set it, I don't what level of messages was the default on
> > installation.
>
> Just drop the dmesg command you added.

Wait, there is something wrong here. You asked me to 'Run "dmesg -n 8" once'.
So I run dmesg once, and restarted computer. I didn't add it anywhere.