I have been experiencing a serious problem since upgrading to Mac OS X 10.4.2 on my 15-inch PowerBook G4 (1.5GHz, 512MB RAM.) It tends to begin while surfing the web. Safari will suddenly stop loading a web page and when I relaunch Safari, it will bounce in my Dock until I force quit. When I force quit, the Finder will go completely unresponsive except for the mouse cursor, at which point my only option is to hold down the power button to force a restart. Upon restart, everything will work normally for another hour or so and the problem will begin again. I thought this was limited only to Safari until today when the same thing happened in QuickTime Player (refused to open a file, bounced in Dock until force quit, force quit froze the Finder and I had to force a restart.) This problem started a week or so after upgrading to Mac OS X 10.4.2. My PowerBook has already been repaired under AppleCare once for a faulty logic board. Is anyone else experiencing this problem? Suggestions would be great.