I have a very similar problem. Occasionally, my computer will hang for a a few seconds, other times it will freeze up entirely. SysRq + R and CTRL + ALT + BACKSPACE do nothing. I remember seeing a similar bug report here, but it's not very helpful: http://www.nvnews.net/vbulletin/showthread.php?t=87692
GeForce Go 7900 GS
X server version 1.3
I've seen this occur on compiz, beryl, and compiz-fusion. I can't remember exactly what kernels I've had it occur on, but I know it has been happening to me since Ubuntu 6.10.