Picture is too large. Heads ate cut of at top of screen and bottom screen is also cut off.

SOURCE: Picture too large for screen

If the television is connected to a cable box such as Time Warner's HD box, the picture mode can be changed by pressing the '#' on the cable box's remote keypad (where the channel numbers are). This cycles the picture between zoom, normal, and stretch which could be cause of your problem.

SOURCE: Mitsubishi WS-55513 55 in. - Picture to latge for scheen

Change your Display Format (widescreen mode) . You're using ZOOM. Press FORMAT on the TV remote control to cycle through the available display formats.

SOURCE: Phillips flat screen 27 inch screen cuts off top and bottom

horizontal over scan , one H.V. cap is out of tolerance in hoz deflection
Fair Cost to fix 75.00 could be a few leaky diodes to. in addition to the cap

SOURCE: Picture too large for screen

Press "wide mode" on TV remote until it is on "full". Check for same on set-top box.

SOURCE: screen cuts off top of heads

Check the menu settings for the picture. You may be expanding the image to fill the sides, in the process, cutting the top and bottom.