iTunes keeps changing sync settings!

I'm trying to remove all songs from my iPhone 4, because they're not playing and I heard redownloading them helps. Some have the red circle and square and newer ones that haven't been purchased and are just on my iTunes aren't on the phone at all. Anyway, I keep unchecking the "sync music" option but it's automatically rechecking it and keeping the same music on there. Has anyone had this problem?