Watching iTunes screen dark and light

I am using an iMac 21.5, Late 2015 iOS 10.13.6 Built-in Retina Display. Each time I watch any movie or TV show on iTunes (version 12.8) my screen keeps going really dark. If I move my mouse the screen image will go to the extreme of very bright with no darks and shadows. If I click on the screen it will settle down and then go really dark soon after. My screen only does this while watching content on iTunes. Watching other content on the web or YouTube is good. I have tried restarting my computer with no luck. Has anyone else experienced this problem? Please help. Thank you.