(In reply to Peter Draganov from comment #9)
>  - What clients were running at the time the issue occurred
> Skype, System Monitor, Firefox, Mate Terminal, Libre Office, Caja, TigerVNC,
> Virtualbox, Pluma
>  - Any special x11 client that can reproduce?
> have no idea
>  - Any particular action that triggers the issue?
> Usually crashes during the night, when a black screen saver is running, but
> it happened several times during the day, when I worked
>  - which desktop environment/window manager?
> LightDM/MATE
> 
> Basically, anything that could help reproducing...
> Just leave X running long enough time - usually after 2-3 days without
> restart

what GPU do you have? I am especially interested in the amount of VRAM it has.