Help- iTunes 4.9 losing music


I have had a problem with the mysteriously appearing exclamation marks for some time, hasn't been too bad though as I could usually go to my back up (separate HD) and get the files if I couldn't find them in the current folder. It was also no more than 1 or 2 at a time.

Today I logged on and suddenly there are about 20+ songs with the exclamation marks (husband made a new playlist today which I think uncovered a bunch). I clicked on each one and tried to match them up to the files, and even using spotlight some of the files are GONE! The weird thing is the songs still play on the ipod.

Why does music keep disappearing? Why is it now totally gone instead of just iTunes unable to find it? How is it still on the ipod?

I am not very comfortable with iTunes losing music like this. Is this a recent bug or a longstanding problem?

FYI 30GB ipod photo with most recent firmware upgrade, Mac OS X 10.4.2, iTunes 4.9