*Inconsistent* error message: requires HDCP playback....

I'm having an issue with my Apple TV that's really bizarre in its inconsistency.

At various times, I will get the following error message:

This content requires HDCP for playback. Either you HDMI cable isn't properly connected or your HDMI connection doesn't support HDCP. Please reconnect the HDMI cable from your Apple TV to your HDTV and try again.

HERE ARE THE CIRCUMSTANCES WHERE I (SOMETIMES) GET THIS MSG

-When I'm at the Apple TV main menu and try to play video (via YouTube page or PBS page, etc)

-When I try to Air Play from my iPhone or iPad.

Sometimes in these two cases, it will play up to six seconds of the video before the error message appears.

The above occurs regardless of what kind of video I try to play (live, on demand, YouTube, sports).

Also, this comes and goes. Sometimes, I have problems in these circumstances and sometimes I won't. Sometimes it will go back and forth even within the same ATV session (without re-booting or whatever).

HERE ARE THE CIRCUMSTANCES WHERE THIS MSG NEVER OCCURS AND WHERE EVERYTHING WORKS FINE

-When I'm at the Apple TV main menu and try to play music.

-When I try to Air Play anything from my MacBook.

STEPS I'VE TAKEN

-Unplugged and replugged in all cords from ATV device to HDTV and power outlet.

-Reset settings.

-Restore factory settings.

-Rebooted cable modem and Wifi.

As I said, the inconsistency of this is bewildering. Tonight it worked. Then I switched to another video and it gave the error message. Then a few minutes later, without me doing anything, it worked again.

Any insight would be welcome.