The signal strength at your aerial doesn't remain constant. It's affected by the 11-year sunspot cycle, individual flares on the surface of the sun, atmospheric pressure and precipitation, harmonics from radio transmitters (such as police & taxis, and even your computer's router) and interference from other electrical devices (such as your central heating timer or your next door neighbour's TV).
The symptoms you've described are consistent with the incoming signal strength intermittently dropping below a level which your TV set can handle. You should theoretically upgrade your aerial (and the associated cabling) but, in practice (given that the signal is OK most of the time), a signal amplifier will probably fix the problem. Asda & Tesco sell them for under a tenner but this model, from Maplin, provides a greater level of amplification:
http://www.maplin.co....gle&u=223504&t=module
Chris