Are you using an HD antenna to get those signals?
Our local TV stations ran a test to determine if your tv was ready for the upcoming change to digital, my new PN50A450P 50" plasma failed the test. My signal was from an off air antenna. What bothers me is that I have another TV on the same antenna that has a converter box and it passed the test on the same channel and had a very good picture.
I called their help line and was told that I had to switch my TV to digital either through a switch or through the menu. I had already done the auto scan and all the DTV channels were showing up in the listing.
My signal comes through a Dish Network receiver (Not HD) and I have a DVD player. Any thoughts on what may be wrong?