When I had this picture freezing problem it was because the signal was TOO strong. The regular engineers could not fix it and had to get Network to fit a new part (amplifier?) in the Cab.
It did take 2 engineers visits before it was correctly diagnosed though. The first engineer just needlessly changed the stb and zoomed off leaving me to wait for the channels to appear.
On the plus side he did change my Pace for a Sammy though.
