I just got cable and now have this problem too; please help.
It's a Motorola DCX3200 hooked up to a Yamaha RX-V663 via toslink.
Originally I was using component (now HDMI) and it was happening so hdmi canceling out is not the reason.
Happens on basically every channel (those broadcasting with 5.1 and also those with just R/L), with no video dropouts.
If I hookup with analog R/L from the cablebox there are no dropouts. Also, if I use the TV speakers; that are getting a signal off the HDMI(digital audio right?)); there are no dropouts.
The input on the RX-v663 was used with dvd player for digital 5.1 surround for years with no dropouts, so that specific input is not a problem.
Seems like some kind of incompatibility between receiver and cablebox?