Music Wont SyncFiles Start Disappearing iPhone 3G

I have an iPhone 3G 16GB with 3.0 and a new iMac with Leopard 10.5.7. Lately I have been having issues, when I sync my iPhone with iTunes it gives me an error saying iPhone not authorized. Then when I go to play a song in iTunes, they all say Locate. When I go to my External Hard Drive where they are stored, the files are all missing. My hard drive still shows the same amount of space free but the files/folders are missing. If I unplug my iPhone and restart my iMac the files are there again and I can play my iTunes music but if I plug the iPhone back in, the issues presents itself again....
How can I fix this? I restored my iPhone but it still does it.
20" iMac Intel Core 2 Duo Mac OS X (10.5.7)