How to repair iTunes library with a lot of "dead" songs?

I have a large iTunes library. I'm seeing a lot "dead" songs - as flagged with a "!".

The songs seem to be fine - they are right there in the HD where they should be, and if I add them back to the library, I get a new copy, and can delete the song with the "!".

However, this is a lot of work. How do they get "dead" to begin with? Is there a way to "rebuild" the library, which would presumably fix these? Or do I need to "fix permissions" or do some sort of repair on the drive? Right now, I use it via Airport, so I can't use disk utility. Its a USB drive, so I'm not even sure I can repair it even if I connect directly to my MacBook?

Any suggestions, other than hunting down each "dead" track and adding it to the library again?

Thanks-
Mark