Never heard of something like that happening. I played it on an SDTV and HDTV and none had any problems, and certainly not the problem you mention.
If you've got your HDTV connected via HDMI, then the cable is not the problem (since digital signals are 0 or 1, no in-betweens). Check other games now and see if they present the same problem. Try to put a 720p game since that's Demon's Souls definition.