Well..I don't know the exact model of Samsung you have. But I just replaced a Samsung 48 inch LED that would change the brightness level automatically no matter what I did. Apparently this was hard locked into the TVs for energy usage compliance ratings. Anway, it was discovered that if you changed the input source to be PC or rather on my particular model, I could change the input name from HDMI..DVD...Satellite...etc and just changed it to PC that it would stop the auto screen dimming feature. No one ever found out why and Samsung doesn't acknowledge it as a defect since it is designed this way by default. Usually disabling all power ECO settings will also stop this, but on Samsungs or at least the one I had, it never did until i changed the Input type to be PC.
Which input are you using for your Atari? I just realized that since all must stuff is AV modded, I do believe I still had this issue through the COAX RF off the TV and wasn't able to change that one to prevent it. After AV modding I was able to change my HDMI inputs etc to all just say PC to stop the auto dimming.