lcd signal problem

I have a westinghouse 37" monitor that when I turn the tv off then on there is no picture just snow screen but has sound. If I turn Comcast box off then on I get the picture. I have also got a few error messages that said the signal has been corrupted and that I should hook it up by component cables not the dvi that it is on. Does anyone have any ideas Comcast thinks its the tv and Bestbuy thinks its Comcast.Thanks bob