Well, have you tried (while connected to your HDTV:
~ # Xorg -configure
~ # X -config /root/xorg.conf.new
If it works, copy the settings from the new xorg.conf to your current one.
If your running Knoppix or Gentoo, you can always install ddcxinfo-knoppix to find out the HorizSync and VertRefresh. The sources may be available on the Knoppix site, but I run Gentoo so I can just:
# emerge ddcxinfo-knoppix