Odd X problem with FC4

[Alan]

Every once in a while when using Firefox or Mozilla, X stops responding
to any input.  The mouse cursor moves, but is stuck in whatever form it
was in when it got stuck.  I can shutdown without problem. (Using the
power key.)  Keyboard input does not work.  

I am not certain what is going wrong here.  I cannot find anything
in /var/log/messages that would indicate the cause.

I am using the x86_64 version of FC4.  I am using the nVIDIA commercial
driver.  (I have to if I want to use the console again without rebooting
or have any graphic speed at all.)

This is on an HP zv5000z laptop.  (AMD64 3700+ with 1 gig ram and an
nVIDIA 440go video.)

Any ideas what is going wrong?  I would log in from the outside during
the lockup, but the way my wireless is configured, that is difficult.

This is pretty frustrating.  It did not happen with FC3 or any of the
FC4 test versions.  (This is a clean install FC4.)

Thanks!