Sounds like your TV could benefit from optimizing its video adjustments. There are some great Video Calibration DVDs that can walk through the process of adjusting your TV's video settings. Or places like Best Buy can send a tech to make these adjustments for you. Also, check your TV owner's manual. Most modern HDTVs have some video presets like "Cinema" or "THX-certified" that will provide the best video quality.
Also, I trust you're using an HDMI connection?
What model DirecTV Receiver/DVR do you have?
What model HDTV?
Is this happening on all channels? Which ones?