Whatever was happening is now worse, compounded by hard reboots and corrupt directory, filesystem, prefs, journal, and cachs impacted.
The way out is to have a working backup, run Disk Warrior after you delete the cache folders and extensions.mkext, turn off journaling (booted from another drive). Then do a Safe Boot, turn off login items until resolved, and move your ~/Library/Preferences to another location or rename to Preferences.backup. The same may be true for ~/Library/Application Support or other folders. (I would not create a test account or add anything until fixed).
And yes, it could be a USB cable or device. Rull out everything first, then add back until it happens I guess (but don't hard restart, try to launch only the terminal to kill processes, or if you must, Activity Monitor, or, just have the FORCE QUIT window.
Do delete any Finder preference though.
Can't diagnose if it isn't stable enough to work on.
If you don't have an emergency backup or boot drive, create one. Then off load and backup and start with repairs.
When you do have a stable system, create a backup image for safe keeping that you can "drop in" and restore when needed. A basic system will fit in 24GB partition.
You can also - if you install OS X multiple times, use a small 10GB partition on FireWire for even quicker installs once your Restore and drop in the Leopard DVD image there.