It's due to over-compression by the cable/satellite company. It's a little different in MPEG2 than it is in MPEG4, but that's what it is. Low signal strength will not do this, it'll just shut down the receiver.

You can count on your provider to continue this practice, they all do. They use system-wide compression algorithms that compress all channels according to total bandwidth. When a bunch of channels all hit high bitrates at the same time, you get the breakup or dropout. Personally, I see it a little less on the MPEG4 channels, but it still happens. Generally it will happen at scene changes.