Had this issue using Amazon Prime. The error on the Apple TV would show

'Apple TV is having trouble maintaining a stable HDMI connection' 

Resetting the devices and power cycling did not fix the issue

Some people have fixed this by unplugging everything, power down, reconnecting and powering back up, this did not work for us

youtube and other apps video worked just fine
when we would test with Prime, it would have sound and no video. If we would fast forward it would show the small image in the FF line. 

Tried different EDID Settings, HDCP settings, rebooted everything.. only thing that worked was updating the FW on the NVX's 

The NVX's were on a 2.xx Firmware so I updated to the latest 7.xx, or latest