Probably nothing - maybe...
The set works fine on channels likely to have a good strong signal - some channels - you do not say which channels - are poor - this will be because of a weak signal - the electronics not being able to make proper sense of it.
Either the weak signal will be down to defects in the aerial and cabling to the set or - which is more likely - down to a weak signal from the transmitter.
Not all broadcast transmitters send out all the channels - your bad channels my originate from a station much further away than the station of the good channels.
The set is fine - some signals are weak.
You might do well to discover your nearest TV transmitter and find out what it's sending out - chances are it is not bothering itself with your weak channels - they will probably come from far away.
Good luck..., July 2015
