Missing songs all the time I update iTunes version

Hi,

Hope someone can help me...

ALL the songs I have in iTunes I BOUGHT from Apple.

Yet I have come to dread updates of iTunes because all of a sudden songs are missing or won't play. I click on the song and it says "this computer is not authorized to play this song" when IT IS AUTHORIZED (was playing it an hour before the update). I do the authorization AGAIN, but still some songs won't play. (They are just as authorized as any of the other songs. Like I said, I BUY them from iTunes store)...

THEN all of a sudden I'm missing 400 songs in the iTunes folder. From 2,000 to 1,600! So I can't even put them back manually.

It doesn't seem right that if I bought all that from iTunes, I should 1/ lose songs and 2/ not be able to get it all back?

400 songs could be 400 euros I spent in vain!

I didn't pay Apple for any special service to keep my songs, or sync them, or anything, but I paid for the danged songs...!

I'm just wondering why I keep hemorrhaging songs like this. This is PARTICULARLY galling, considering I'm forced to update the App every now and again and then find I can't play songs I paid for.

(And what is available in Purchases to download is only a few songs that are recent).

Sorry that this is long, but I wanted to leave it clear that songs DISAPPEAR, from my library (won't play) and from my iTunes backup (obviously not external).

THANKS!