Digital HD requires better signal level than analog and the frequencies are almost all in UHF now. the existing channel numbers are basically "place setters" (why the TV has to scan these in on setup).
I changed my antenna out to a true HD antenna, made a world of difference. Try one of these:
Antennas Direct | DB4 The Best HDTV Antenna on the Market
They also have an 8 element if your way out in the boonies.
BTW if any of you get flack from your local natzies, I mean housing board. There's an FCC ruling passed in '96 that says you can go up to 12ft above your roof line. They can't tell you to take it down. Here's the rulling:
FCC Fact Sheet on Placement of Antennas