Lately I've noticed that my G5 dual processor tower is freezing when I shut it down. The desktop goes to the blue screen, but it remains blue and I have to manually shut down by holding the power button. I've also noticed it has been slow to start up. Most of the times it starts up if I wait long enough, but sometimes it hangs up during the startup process also. Is there anything I can do to alleviate this situation? I am running OS X Version 10.4.5.