Horizontal black lines and "ghosting" appearing in iMac screen

Yesterday my iMac started displaying these horizontal black lines across the middle of the screen, as well as some image "ghosting". I've tried numerous fixes, including leaving the computer off for hours and upgrading to the newest OS, but the lines still persist. Obviously this is not ideal as it makes the screen look rather garish. I see no option to change my monitors refresh rate and also cannot seem to reset the NVRAM (holding down Command, Option, R, and P at start up seems to do nothing and the computer continues to boot normally). This is very frustrating and I'm hoping isn't going to result in a hefty service charge if it needs to get fixed. Any advice would be welcome. Thank you.