Motorola HD Cable box
With TV on, turn the cable box power off
Push the Menu button on Remote or Box
Wait several moments for a User Settings menu to appear on screen. (It comes up, even with power off)
Using cursor keys, navigate to the HDMI setting. It probably says 720p next to it. (if it says 1080i, that's the max. Not many, if any, stations are broadcasting at 1080p)
If it's 720 or lower, press the OK/Select button. Screen will blank out and reappear with new setting. Every time you press the button, it jumps to a new setting. Do it until you get the highest your box offers.
Once you have 1080i or 1080p showing, turn the power back on with the power button. You should now have a better picture.