You need a better aerial (or, at least, a signal booster).
In an ideal world the lengths of the elements of a TV aerial should be exactly matched to the frequency of the incoming signal. Since every channel is on a different frequency you theoretically need a different aerial for every channel you watch. (Imagine that lot on your roof!). Clearly that's not possible, so we use aerials which only provide a 'rough match' to each of the frequencies used by broadcasters. That inevitably means that the 'match' will be better with some channels than with others. (Your aerial is weakest at 'pulling in' ITV1, C4 and Five).
Signal strengths at your aerial don't remain constant. They're affected by atmospheric pressure, weather conditions, the time of day (because the reflective layer of the Earth's ionosphere varies with the amount of sunlight), the 11-year sunspot cycle and individual flares on the surface of the sun. Your aerial can 'pull in' all of the different frequencies for most of the time but when signal strengths drop it can't do the job with those frequencies which it's poorly matched to.
A simple signal booster might well fix the problem. If not, you'll need an aerial like this one:
http://www.maplin.co.....aspx?ModuleNo=221124
Chris