Lines will not go away until you reimport the image.
Thanks I will certainly try that. it does not happen to all files. I work at a school and these issues are occurring in our digital arts lab on about 4-5 macs. It seems pretty random when the issue does occur. I know there is an OS X update for those machines so i will try to update them a little later to see if that helps.
Yes. And in fact after restarting photoshop the student opened another picture he was working on and that became corrupted with missing layers. Flattening the layers didn't resolve this as well. Things have been working perfectly all school year until that last couple weeks evidently.