A bit of an update:

The beta firmware seems to have fixed my "change channel and totally lose sound" issue, but the other issue remains.

Set the cable box on a channel, watch it for a few minutes at a set volume. Then, change volume up or down: Volume spikes up or down, then becomes very quiet. Turning it up at this point, or down during the spike, have no effect.

I still hope to be able to talk to tech support tomorrow, so hopefully I can find something out.