Late last night I tried to do some work on my G5 iMac. It has a large external firewire drive attached. Everything was slow, and the CPU was pegged at 100%. The firewire drive was making continual noises, and when I looked in the hierarchical view in Activity monitor, it showed Perl, associated with Cron, taking up most of the CPU.

I assume this was some nightly cleanup task going on, but it has never been this disruptive. I unmounted the drive, and the symptoms disappeared. Only difference has been the recent upgrade to Tiger 10.4.9.