This is a classic symptom of insufficient signal strength (or of an interfering signal making it difficult for your receiver to resolve the primary signal).
If you live near an airport (or anywhere else where signal paths could be affected by intermittent 'bounced signals' ), you might be suffering problems associated with passing planes (or other types of vehicles). However, it's far more likely that your aerial simply isn't up to the job. Television signal strengths don't remain constant. They're affected by weather conditions, the heights of ionospheric layers (which vary with the seasons) and the sunspot cycle on the sun's surface. (Either way, you need to consider an aerial upgrade).
When Freeview was introduced, it was estimated that approximately one in three households would need to upgrade their aerial systems. ('System' means the aerial
and the cable). In practice, it seems that fewer people need to change their aerials but it's certainly not unusual.
First, try a cheap signal booster (from Tesco Extra, Asda or Maplin Electronics). If that doesn't work, get hold of one of these
http://info.maplin.co.uk/module.aspx?moduleno= 22523
and have it professionally installed, together with new cabling.
Chris