Comment 3 for bug 581549

I have a series of HP machines all using the Vidia Corporation NV6 [Vanta/Vanta LT] (rev 15) as the video controller

These all hang X after a random period of time after start up whether or not X is being used actively but if you do something that manipulates the screen then it is more likely to happen within a few seconds. If you dont log in or use console VTX (ctrl-alt-f2 etc) then the PC stays running.
In some instances you can ssh into the machine after X has locked up.
syslog shows many repeats (several per second) of
nouveau 0000:01:00.0: PFIFO_DMA_PUSHER - Ch 1

These machines were fine on Karmic but fail on fails on Lucid and Maverick Alpha2 (when nouveau is the default) and is worse when using XFCE. It is irrespective of upgrade or fresh install. It is marginally more reliable if using Gnome (These are low spec machines so xubuntu/xfce is my prime choice for these machines.

Dropping back to VESA (setting nouveau.modeset=0) (Not sure why it didnt drop to nv instead but thats a different issue) the machines are stable.