I have a MAC G5 that will freeze on startup or decide to freeze in the middle of something important. When this happens, internal fan speed gradually goes to max and makes a lot of noise. I have manually reset the SMU and PRAM and ran the disk utility to repair permissions. Permission repair did fix the problem for a while. This is a new OS 10.4 install. My take is that something is failing and overheating making the fans do overtime. Any ideas? I think I am going to try a new hard disk next.