21.5 mid 2010 iMac dimming and then going black

My iMac (21.5-inch, Mid 2010) screen is flickering then goes black. If I shine a flashlight to the screen, I can still see everything and am able to navigate to shut down. Sometimes, it will work fine for a few hours and then it will go back to doing the same. It seems to last longer if I keep the screen dimmed. However it still acts like it's adjusting the brightness (it's not set to auto adjust.)