Are you using the HDMI output or the component output on the HR20?
If you're using the component output, it could be due to a slight sync timing mismatch which can be fixed by using your TV's horizontal and/or vertical positioning setting.
I've got a HR20 as well and the times I've seen what you're describing is always because of sloppy digital frame buffer use by the station in question from upsampled non-HD origin programs.
Their paintbox or effects box is parking "off-frame" information slightly on-frame which results in the artifacts being visible. Sometimes what you're seeing is VITC (Vertical Interval Time Code) information. This is a bad habit in analog production as traditional analog TVs have overscan which effectively hides these items.
I actually use my HR20 mostly on fixed resolution set to my display's native resolution even though I have a specialized scaler.
The reason for this is DirecTV is a digital signal source, so the receiver is supposed to image the MPEG-4 or MPEG-2 data stream into the selected size of the output frame size.
This often will result in a better quality picture than having the digital stream unpacked into a size and then scale the resulting picture to your display's native resolution, especially if you're using component outputs instead of HDMI.
This is also the case for good upscaling DVD players, the scaling happens at the decoding stage rather than at the picture stage.
That said, there are some situations where "line-level" scaling can provide a more visually appealing result as many advanced scalers know how to prevent moire artifacts while many imaging routines don't account for moire.
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