Once every day or so my system practically hangs (gets extremely unresponsive;
X unusable; only SysRq magic working) while using Xorg with the nouveau driver
(version 1.0.10).

I am running Debian GNU/Linux testing (jessie) with

$ uname -r
3.13-1-amd64
$ lspci | grep VGA
05:00.0 VGA compatible controller: NVIDIA Corporation NV40 [GeForce 6800
GT/GTO/Ultra] (rev a2)

This problem did not occur when I ran Debian stable (with nouveau).

When the system (practically) hangs, I get messages like this:

... GPU lockup - switching to software fbcon
nouveau E [Xorg] failed to idle channel 0xcccc0000 [ ...