You need to get a better aerial (or, at least to try a signal booster with your existing aerial).
I can foresee you wanting to challenge that statement because
(a) the problem doesn't occur all the time ; and
(b) it only occurs on some channels.
However I'm 100% certain that your aerial isn't really up to the job. (Picture break up, or the complete disappearance of a channel, is a classic symptom of insufficient signal strength). For the reasons why it only happens at certain times, and on particular channels, please refer to my posts here:
http://www.theanswerb...y/Question858245.html
Chris