So far I have only reproduced this issue while playing youtube videos in firefox. This did not happen with official nvidia drivers (before it got
moved to nvidia-tesla-driver). While playing a video, the X will randomly freeze (sometimes after a few minutes, sometimes an hour). Audio will
continue playing for some time. The system does not respond to any keyboard input (not even attempting to switch to TTY), but mouse cursor continues
to move.

I was able to connect to that PC through SSH from my laptop. The Xorg process was stuck at 100% cpu utilization (one core/thread). I noticed that
there was no extra output in dmesg, or xorg log. I have perf installed, so I ran perf record - this showed that most of the CPU time is spent on
nouveau_dma_wait. File with perf archive output: https://www.dropbox.com/s/l8bsqp0jdyfnvom/perf.data.tar.bz2?dl=1

After killing the Xorg process, the system appears to work normally, but I had one incident where after restarting X, it froze again almost
immediately and it became completely unresponsive, with no ability to connect through ssh, not responding to ping. Since I don't have a serial
output, I can't see what actually happened.