ST2320L - horizontal lines across the screen

I have a Dell ST2320L LCD monitor mated with a Dell XPS system.  I recently ungraded to Windows 8.1 which did away with my AMD Catalyst Control Center (CCC).  I uninstalled and then reinstalled CCC, and CCC is working again.  However, know I have these faint white horizontal lines that run across my screen.  I have unplugged the HDMI cable, reset the monitor to factory settings, etc., but the lines are still there.  I've hit buttons one and four on the monitor and ran through all the colors, but you can't really see the lines. They are especially pronounced on a white page. You can really see them on dark pages or pictures. When one of the lines happens to go through text, the text appears out of focus.

Any idea what's going on?

Thanks!