I am using the DCX3400 right now and have used a DCX3400-M (from another state hence it couldn't receive premium channels) before this. No such symptoms as you have reported. Not even once. Sometimes sound wouldn't go through for the first few seconds when I turn everything on but as I change channels, sound will resume and never be dropped again.
I have no solution for you. But an advice; try to get the new DCX3501 instead.
I did order 2 new DCX3501 boxes should be here in the next day or so!
How did you order 2 DCX3501 boxes? I had an old DVR box and tried to swap mine out for a new box and they won't let you ask for a specific model over the phone. I went to the store to get a newer box but they were out so I had to keep checking by their store.
I had the same issue running HDMI through my Denon AVR-2311 to Samsung 7 series TV. Comcast was no help with the issue. Finally was able to fix it by changing the settings in Motorola to 1080P pass through On and Audio Pass Through On.