I suspect Intego Antivirus 6 is a problem for many systems. I installed it on my G5 Quad (2005 - still works well!), and suddenly would get crashes on full scan (kernel panic), and frequent (at least once a day) freezes that seems to be Safari related (not 100%): system would become unable to start of terminate tasks or threads. Arrow moved just fine, but eventually every thing would be the spinning beach ball of boredom. Hard restart required.
I actually found a repeatable sequence:
In Safari: browse to www.alliedelec.com
search for "littlefuse"
search within results for "3AG" hit return
Blammo. partial blue loading bar in Safari, nothing else works.
Uninstalled Antivirus V6, system no longer exhibits that behavior, and it's faster.
This problem does NOT occur on my MacBook Core 2 duo, nor on an iMac 24" (2010).