We've been so focused on the audio from the cable box that I had to backtrack a bit.
While watching TV through the cable box, the tv goes to black like when the 990 is turned off. Turning the 990 off then on brings the picture back.
Are there any triggers that typically cause this condition? Changing to or from certain channels, for example? Also, when this happens, I assume that the audio continues. Can you change inputs and be OK?
The 990 must also be turned off and on too when switching from DVD to Video 1 (cable), switching from video 1 to DVD is ok.
It sounds like the 990 has to be cycled off and back on when switching to the Video1 input in order to get video to the display. Is that correct? If so, it sounds like an odd HDCP handshake issue of some sort. I'd be curious to know if you can power cycle the TV or cable box and resolve the problem that way as well.
Another issue is a blue screen for a few seconds on the tv while watching cable or a dvd, the audio is not affected when this happens.
If this just happened with cable, I'd suspect signal strength issues, although that would also affect the audio signal. Are there any common triggers such as going from DVD menu to the movie, changing channels, or going to and from recorded content on the Motorola?
When you get a popping sound from the sub, is it at start-up and/or shut-down or does it just happen randomly? It could be related to the sub's auto-sensing on/off circuit. A good test for that would be to simply turn the sub to "on" and leave it on for a few days - if the popping stops, then it's caused by the sub either turning itself on or off and you could either leave it on all the time or think about decreasing the gain on the sub's amp and increasing the sub trim at the 990 so that the input signal is higher. That would make the sub more likely to trigger on sooner and to stay on longer.