I had this same problem, and I didn't use to have it. It turns out that in addition to fixing it by forcing a lower resolution in the Roku display settings, I can also fix it by going to Settings->System->Advanced System settings->Advanced Display Settings, and make sure "Auto-adjust display refresh rate" is OFF. This is on a Roku Premiere stick. I had turned that on "for fun" a while ago and forgot about it. when I do this, ATV+ will play everything, even if the video autodetect that others have mentioned is on.