This is a classic symptom of an inadequate signal from the aerial. Freeview signals tend to be affected by rain and also by wind (which shakes the aerial so that it's not perfectly aligned for reception). The ITV channels tend to be the ones which disappear first.
As your situation is clearly a borderline one, you
might be able to resolve the problem simply by using a cheap aerial booster. However, you might need to get an aerial fitter to upgrade your aerial. The type of aerial you require is something like this much-praised one:
http://www.maplin.co.uk/Search.aspx?criteria=P T18U&DOY=31m12
(If you get the aerial changed, you should ensure that the fitter also changes the cabling).
Chris