What is the signal source that you are watching on your HD61Z456?
The problem you describe usually happens when you aren't using an HD source or have downgraded the signal by the composite input on your TV. (Composite cables only support 480i SD signals.)
You will only see 720p HD content from HD sources, if you use HD compatible inputs. Some OTA channels are also broadcast as only analog signals (usually the -2 and higher sub-channels). If you have cable or satellite, make sure that you have the HD programming and have tuned to the HD version of the channel.
The HDMI and component inputs both support HD signals.
I hope this helps. If you are watching an HD source and the problem persists, I recommend trying new video cables. Next, check if you are getting interference from your devices by using a cable with a Ferrite core. See http://www.monoprice.com/Category?c_id=102&cp_id=10240
for HDMI cables with Ferrite cores. The cores help block interference. If you have very long cable runs, check the behavior with shorter cables between the source and the TV.
(I am assuming you aren't using an SD set top box (VCR, older cable box, low end digital-to-analog converter).)