Odd thing recently happened.
My Apple TV stopped working on my TV - it's 3 years old and has been working fine up till now.
Booked an appointment at the Genius Bar (waited 4 days) and they reset/updated the software and plugged it in inshore to show me all is well again.
Get home and same problem - the Apple icon flashes onscreen with red and white background flashing till the ATV shuts off.
Now if I plug it thru the XBOX and into the TV - voila, it works perfectly.
My guess is the XBOX is taking the signal, changing it to 60hz, and sending it out to the TV (100/120 hz - like most TVs)
And of course it works fine on the PC monitor.
Yes, I have tried both HDMI ports and changed the cables. Thing is, I have 2 of the same TV's and same problem on both.
Like I say - they have been fine for years.
So what gives ? Any guesses ?
Try updating (or backdating) the TVs firmware.