I suffer the same issue.

What I have done is to create two smart playlists: one where album is Unknown, and the other where Artist is Unknown.

Generally this enables me to find the offending tracks and fix them. However, I find I need to re-add them to playlists... So I don't remove the broken link until I have done that as well.

The pattern is random. I've consolidated years ago, etc, etc, etc.... Seems to be a gremlin.

iTunes not accessing library
 

I've been using iTunes in windows for many years. Fundamentally it is a flakey piece of software.

I got my music back into iTunes by importing all my music from my external drive (this is where the iTunes libray is pointed to). This works, but is a very clunky way of doing things.

My backups before reinstalling the iMac have failed miserably to deal with my playlists - I will need to do them again. Ho hum - the wonderful World of Mac :o.

Cheers,

Mike

iTunes problems
 

Try this link: http://www.ilounge.com/index.php, where you might find "Transferring your iTunes Library" and "How iTunes Manages Content Media". One very essential rule is not to touch (alter their name) iTunes library's files, other ways the exclamation point appears!
In case the have canceled the article above, notify me your e-mails, so I can send it to you. (I don't know how to submit it here! Any suggestion?)
Good luck!