Given the new details, it sounds like it may be an OS corruption.
Were you able to verify/repair disk permissions as well as the disk itself? In order to verify/repair the disk itself (which sometimes works to iron out OS corruptions), you need to put your Apple install disk into the machine and boot off of it. To do this, hold down the 'Option' key when you're restarting the machine and choose to boot from the CD. From there, you'll be presented with a screen to reinstall the OS, but you'll just want to select the disc utility icon from the utilities menu at the top of the screen. Verify/repair the disk and see if the problems persist.
If none of these steps (or the ones I mentioned before) seem to work, then you may have to do a clean install of the OS. How old is the machine anyway and when was the last time the OS had a clean install?