G5 Tower - Extremely slow performance in Finder, hanging apps

Question

About the system:
Power Mac G5
2 x 2.5GHz CPU
4.5 GB RAM (physical)
160 GB HDD (85 GB free - formatted Mac OS Extended Journaled)
OS 10.4.11 with latest software updates downloaded and installed
This box is running incredibly sluggish - apps are taking 20+ bounces just to open, as many as 10 to 30 seconds to simply open a folder from the desktop. Even trying to drag a window has a significant delay.
This all seemed to start when I copied to this machine, a backup of my music files library from an external drive that had been attached to another Mac (Intel Mini) - about 50GB of music, approximately 10k songs. I opened iTunes 8.0.2 (for the first time ever on the G5) and it hung trying to add the new files to the empty library. Since then, the entire machine seems to have been hosed. I realize the iTunes thing might be a red herring, but the problems coincided exactly with the new music and first launch of iTunes.
Since then, I've tried pretty much everything I could think of - ran the Disk Utility and OnyX utilities (all were clean), zapped the PRAM - even reinstalled Tiger. Same problems. Activity Monitor shows the CPU running at 98% - 99% idle when this is happening (although it takes 90 seconds to open Activity Monitor)and shows 3.92 GB of free RAM. So, the computer doesn't seem to be overworked.
Any thoughts or ideas would be most appreciated. Unfortunately, AppleCare is no more for this box...

Answer 1

Since the Power Mac seems to be running normally while booted from the DVD, it must be related to the hard drive in some way. If it has 85 GB of free space, it should not be a problem with lack of free space. If it was me, I would clone (exactly duplicate) the internal drive to an external FireWire drive using Carbon Copy Cloner. Then try to restart from the external drive.
If things operate normally while booted from the external, the current system installation is fine. I would then reformat (erase) the internal drive using Disk Utility. Then clone the external back to the internal. See if the internal now operates normally. It should. But if it does not, there is probably something wrong with the drive itself (a hardware problem).
If the problem recurs while booted from the external, there is something wrong with your general system installation. Creating another user account proved that the problem was not isolated to one user. In this case, you have two options. You can reinstall Mac OS X on the internal using the +Archive and Install+ option. This option will retain your user data and settings, but give you a fresh system. In case something bad happens, the clone on the external will serve as your backup. OR you can reinstall Mac OS X using the +Erase and Install+ option. This will give you a completely new installation on an blank volume, and you can migrate your data from the external drive after making sure everything is working normally.