In an ideal world, the lengths of the elements on your TV aerial should be exactly matched to the frequency of the channel which you wish to watch. That would mean that you'd need a different aerial for every channel (or, more accurately, since digital TV broadcasts channels in 'bundles', for each group of channels). That would create a massive clutter of aerials on your roof!
So a single aerial can only provide only a 'compromise solution', where the lengths of the elements are better for receiving some channels than for others. If reception is already 'borderline', that can result in the weaker channels disappearing from your screen.
Theoretically you should upgrade your aerial to something like this
http://www.maplin.co....ital-tv-aerial-221124
and also renew the cabling at the same time. However, since your aerial is clearly almost good enough for the job, a simple signal booster (£9 from Asda) is likely to solve your problem.
Chris