We have a G5 dual 2 running 10.4.7 has been having this problem. On start up it does and doesn't make it to the desktop and freezes. The fan then continues to wind up faster and faster. At this point you panic and reboot the machine. It is then fine after that. It has happened several times now in the morning at start up. The previous night it was shutdown properly. Any one have an idea of what is going on?