I've had system lock-ups on FC 12 x86_64 ( 22.214.171.124-174.2.22.fc12.x86_64 #1 SMP Noveau driver and Nvidia graphics) By "lock-ups" , I mean that the system was so unresponsive that the numlock key didn't respond and CTRL-ALT-F2 didn't bring up a console. I've also had peculiar behavior where the system responded slowly with much hard drive activity, as if it had suddenly run out of memory and was swapping every time I did something.
I think these problems were caused by using Firefox. I switched to Opera and haven't had them again (yet). The /var/log/messages log was showing kenerl "oops" messages, as posted in the thread: http://forums.fedoraforum.org/showthread.php?t=240530
Have any of the participants in this thread see similar messages in their logs?