I had this problem last year starting with channel 4 and 5 so eventually the only way I could watch anything was with subtitles. After a lot of phone calls we reluctantly agreed on having the engineer come out and the box was replaced. We looked into similar problems on the web and the most popular theory was that the older boxes could'nt cope with the automatic software upgrades for the HD boxes. We mentioned what we had read about to the engineer and he more or less admited that this was a problem. Our box was replaced and we have had no problams since