Samsung 610 46'' lcd - hdmi input not wor...

just bought a samsung 610 46'' lcd Motorola HD cable box connected to HDMI/DVI input 1 on tv. was working fine out of the box, but this am, tv said signal was weak (720x480) instead of the hi-res signal. screen shows signal for about 3 secs and then goes black. i tried other hdmi inputs and video works fine, but no audio because i am using an hdmi/dvi cable. is this a software issue on the tv, or do i have a bad hdmi input. seems like the cable and cable box should be fine, because the hdmi video worked on one of the other hdmi inputs.