Quicksilver 2002 won't shut down. . Need help.

I am using the 933MHz. version. Have 3 internal HD's, 2 external firewire HD's. I have two USB PCI cards installed. Memory is 1.5 GB. I had the exact same machine,which did this. I finally gave up and switched to my spare. Changed over the PCI cards, HD's and memory. Same issue... I've tried the usual ways to isolate the problem, but none seem to work. i.e. - Replaced battery, predded and held pram button, and started up using key combo to make it chime three or four times. Still no luck. I unplugged all USB and Firewire cables except keyboard, removed the USB cards one at a time, as to preserve keyboard use. - Nothing. I also repaired privileges. This just happened out of the blue. I hadn't added any Apps, cards, or anything else. I would be thankful for any suggestions. Tom