Most likely you have a refresh problem. You should start setting your resolution in the computer to 640x480 at 60hz refresh and work up from there. Be sure to lock your refresh to 60hz. Also set your driver to send the video to both your monitor and the TV while getting set up.
Also you can run the Moninfo to find out what resolutions your TV will accept. See: Hooking Up A PC to A HDTV
for more info and links to get the moninfo software.