It could be a handshake issue, however when that occurs, you usually get the opposite issue - no video, and audio instead. This is an issue with HDCP - and the box is basically telling the television that it's not going to send a signal until it can verify it is a panel. Now whether this is a box issue or not is something I can't diagnose from here.
The fact that turning off the cable box leads me to believe that this is the source of the issue, but at this point, I can't be positive. I would suggest that updating or resetting the box might work, or replacing it may be an option as well.
Keep me posted.
I am having an audio problem with my 850. When I turn on TV and cable box (SA8300) I'm not getting audio. I have to turn off cable box-then turn back on and audio works. This is my second 850. I returned the first one due to clouding/flashlight issues but I did not have the audio problem. Could this be an HDMI handshake issue? I have the current firmware (1005) so I don't know what to try next. Any suggestions?