Comment 17 for bug 6279

I have this issue too. I can confirm it is a Xorg issue, since if I am running Xgl, I have NO problems at all, but as soon as I switch to Xorg I get the lockups.

I have this problem on my two computers, one being a PIII 600Mhz with a Geforce2 GTS (nvidia-legacy) and the other a P4 2Ghz with Geforce2 MX400 (nvidia-glx). Running with the nv driver I had not experienced this problem yet.

It seems to occur at random, not related to any specific app. I had seen this occur mainly on Firefox and Nautilus tho.
The freeze seems to lock the keyboard but not the mouse inputs, it also locks the whole desktop, but pressing the shutdown button on the case causes the system to cleanly shutdown. I have also seen some instances where it completely locks the whole system, tho more rarely.