Loss of ITV digital channels on Sony KD-28DL11U?

Can anyone tell me why I lose digital ITV channels 1 & +1, Channe 4 +!and Channel 5 but not ITV3 and not the analogue signal. This happens after the set has been on for perhaps an hour at lunchtime. The signal returns late in the evening. I have checked with ITV as suggested by the Digital advice line and they say it could be outside interference but a set in another room with set top box is not affected. A local TV repair company on the phone said it was the old set and get a digital box. The picture quality is very good always on BBC channels and on ITV,when it is available. The picture first breaks up and then the screen goes black with just a banner across the top saying the channel either is not available or it is not tuned in. It changes from one to the other. Can anyone help please?